UK Certificate of Sponsorship Explained: Everything You Need to Know

A Certificate of Sponsorship (CoS) is one of the most fundamental documents in the UK work visa process, yet many applicants and even some employers don’t fully understand what it is, how it works, or why it matters so much. This guide covers everything you need to know about the CoS, from how it’s issued to what happens if something goes wrong.

What is a Certificate of Sponsorship?

Despite its name, a Certificate of Sponsorship is not a physical certificate. It is an electronic record — essentially a database entry in the Home Office’s Sponsor Management System (SMS) — that contains key information about a job offer and the worker being sponsored.

When an employer “assigns” a CoS, they’re creating this electronic record and providing the worker with a unique reference number. The worker then uses this reference number when applying for their visa.

Think of it less like a physical certificate and more like an authorisation code that links your visa application to your employer’s sponsorship record.

What Information Does a CoS Contain?

A Certificate of Sponsorship includes:

Worker’s personal details: Full name, date of birth, nationality
Job title and SOC code: The occupational classification of the role
Start date and end date of employment/visa
Salary: The confirmed salary for the role
Sponsor’s licence number
Unique CoS reference number
Visa route: e.g., Skilled Worker, Health and Care Worker
Working location(s)
Whether the role is in a shortage occupation
Whether the applicant qualifies as a new entrant

The information in the CoS is binding — it must accurately reflect the actual job being offered. Discrepancies between the CoS and other application documents can cause delays or refusals.

Defined vs Undefined Certificates of Sponsorship

There are two types of CoS, and understanding the difference is important for both employers and workers.

Defined Certificate of Sponsorship

Used for: Workers applying from outside the UK, or switching from an immigration category outside the “switching” rules

Key characteristics:

Allocated on an annual basis from the Home Office
Employers must apply for a specific number at the start of each year
If the annual allocation is used up, the employer must apply for more
Has a 3-month validity period from the date of assignment
Must be used within 3 months to make the visa application

How it works: Each year, UKVI forecasts demand and allocates a pool of defined CoS to licensed sponsors. Large employers typically request and receive several hundred. When they hire someone needing a visa from abroad, they draw from this pool.

If an employer uses more than their allocation, they must apply to UKVI for additional certificates. This process takes time, so employers with high volumes should request more than they expect to need at the start of the year.

Undefined Certificate of Sponsorship

Used for: Workers already in the UK who are extending their visa or switching from an eligible immigration route (e.g., Graduate visa to Skilled Worker)

Key characteristics:

No annual allocation — employers can create these on demand
Generated directly in the SMS without needing a pre-allocated pool
Also has a 3-month validity period once assigned
More flexible for employers managing in-UK recruitment

If you’re already living and working in the UK on a Graduate visa or another eligible route, and you find a sponsored job, your employer will assign an undefined CoS.

How Does an Employer Assign a CoS?

The process is managed through the Home Office’s Sponsor Management System:

Level 1 or Level 2 User logs into the SMS
Selects the appropriate CoS type (defined or undefined)
Enters the worker’s details — name, date of birth, nationality
Specifies the job details — title, SOC code, salary, start date, working hours
Confirms the role qualifies for the relevant route and meets salary requirements
Assigns the CoS — this generates the unique reference number
Shares the reference number with the worker

The worker must apply for their visa within 3 months of the CoS being assigned. If they don’t apply in time, the CoS expires and a new one must be assigned.

CoS Validity: Key Timing Rules

For the Worker’s Visa Application

A CoS is valid for 3 months from the date of assignment
You must submit your visa application within this 3-month window
If you miss the window, your employer must assign a new CoS

For the Visa Itself

Your visa start date cannot be more than 3 months before the job start date shown on the CoS
The visa end date is typically based on the job end date (if fixed term) or set to a maximum of 5 years

Important: CoS Cannot Be Reused

Once a CoS is assigned and used in a visa application, it cannot be reused. If a visa is refused, the employer must assign a fresh CoS for any new application.

What the CoS Means for the Worker

When you receive your CoS reference number, it means:

Your employer has formally committed to employing you under the stated terms
The Home Office has a record of the sponsorship arrangement
You can proceed with your visa application
You should receive the position at the salary and under the conditions stated in the CoS

Important: The CoS is not a guarantee of a visa. It’s a necessary document but the Home Office still assesses your full application.

What Happens to the CoS if Things Go Wrong?

Worker Doesn’t Start the Job

If a sponsored worker accepts the CoS but then doesn’t start the job (for any reason), the employer must report this to the Home Office within 10 days through the SMS. The CoS becomes void.

Worker Leaves the Job

If a sponsored worker leaves their employment (voluntarily or otherwise), the employer must report this within 10 days. The worker’s visa may then be curtailed, and they will typically have a 60-day window to find a new sponsor or leave the UK.

Employer Loses their Licence

If the employer’s sponsor licence is revoked, all associated CoS become invalid. Workers sponsored by that employer typically have their permission curtailed and are given 60 days to find alternative sponsorship or depart.

Visa Application is Refused

If the visa is refused and the worker wishes to reapply (or the employer wants to reapply for another worker), a new CoS must be assigned.

Transferring to a New Employer

When a Skilled Worker changes jobs, the new employer must assign a new CoS. The worker then applies to update their visa to reflect the new employment.

Key points about job changes:

The new role must also meet eligibility criteria (salary, SOC code, etc.)
The worker cannot start with the new employer until the visa update is approved (unless their current visa allows work for any employer)
If the worker switches employer without updating their visa, this is a breach of conditions

Common Questions About CoS

Does the employee see the full CoS?

Workers receive the CoS reference number. The full details are held in the SMS and accessible to the employer. Workers can view some information by entering their CoS reference number on gov.uk.

Can a worker have multiple CoS?

A worker can only use one CoS for a visa application. However, if they change jobs, a new CoS is assigned for the new role.

What if the salary in the CoS is wrong?

If the wrong salary is entered, this must be corrected before the worker applies. The employer can request an amendment through the SMS in some circumstances, but in others must assign a new CoS entirely.

Can a CoS be assigned for a hypothetical position?

No. The CoS must relate to a genuine, confirmed job offer. Assigning a CoS without genuine intent to employ is a serious breach that can result in licence revocation.

Does working from home affect the CoS?

Working location should be accurately recorded. If a worker is primarily home-based, this can be noted. Multiple work locations (e.g., client sites) can also be included.

The Immigration Skills Charge and CoS

When an employer assigns a CoS for most Skilled Worker roles, they also become liable for the Immigration Skills Charge (ISC). This is paid when the worker submits their visa application, not when the CoS is assigned, but employers should be aware of it.

Current rates (from December 2025):

Small or charitable sponsors: £480 for the first year, £240 per 6 months thereafter
Medium or large sponsors: £1,320 for the first year, £660 per 6 months thereafter

Exemptions: Health and Care Worker route, student visa switches, and some others.

Tips for Employers Managing CoS

Request defined CoS early in the year — don’t wait until you have live vacancies
Keep track of your CoS allocation — running out mid-year can delay urgent hires
Assign CoS only when ready — the 3-month clock starts immediately on assignment
Check all details carefully before assigning — errors can require reassignment
Train your Level 1 Users — whoever assigns CoS must understand the obligations
Keep records — you’ll need evidence of why each CoS was assigned for compliance purposes

Tips for Workers Receiving a CoS

Apply promptly — don’t delay your visa application once you have the reference number
Check the details match — verify your name, date of birth, and job details are correct
Don’t start work before your visa is granted — working under a CoS alone is not permission to work
Keep the reference number safe — you’ll need it for your application

Conclusion

The Certificate of Sponsorship is the linchpin of the UK work visa sponsorship process. While it might seem like a bureaucratic detail, it represents a formal commitment between employer and worker, governed by immigration law and subject to compliance scrutiny.

For workers, understanding your CoS helps you ensure the information in it is correct and that you apply for your visa within the validity window. For employers, managing CoS carefully is central to running a compliant sponsorship programme.

Looking for a licensed employer? Search our directory to find UK companies with active sponsor licences who are hiring internationally.