This page explains if you are allowed to do any other work outside your normal hours for which your work permit allows.
A person who has a work permit is able to undertake work, provided:
The work permit holder is not allowed to enter self-employment, set up a business or join another business as a director or partner without obtaining further permission from us.
Under the following categories of the work permit arrangements, work permit holders are not permitted to take supplementary employment. These are:
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Customer Contact Centre, UK Border Agency, PO Box 3468, Sheffield S3 8WA