If someone comes to this country on a student visa but then leaves their course?

and starts working instead are they an illegal immigrant and could they be sent back

Answer:
They can work 20hrs per week only. This right has been confirmed in the judicial review case R vs ZHOU.
This has indeed come up before. I will copy my previous reply:

During vacation time a full time foreign student WHO IS PRESENT ON STUDENT CONDITIONS - NOT VISITOR CONDITIONS - can work full time.
THIS NEXT BIT ANSWERS YOUR QUESTION: A student who STARTS COLLEGE but drops out can still work 20hrs per week even if they are not studying (as confirmed in the ZHOU judgement). They CANNOT lawfully work full time - they will be working in breach.
Any student who NEVER starts school can work 20hrs per week but will be liable for action as an illegal entrant (by verbal deception or leave to remain by deception) if caught. A student who is on VISITOR conditions can do NO WORK at all - paid or unpaid.
The answer to that question is yes as they would only have a student visa and not a working visa. Shop them and geet them sent back.
This question came up earlier in the week - the clue to the answer is in 'Student Visa'. You have to be a student to have a 'student visa' - you were allowed to enter the UK on the status of being a 'student'.

You are no longer a 'student' - you are now illegal and you are not supposed to be working. The one way flight ticket is already waiting for you/your friend etc.
Yes they are illegal, they came to this country to study not work, that is the only reason they got the visa, so that person! is in breach of the terms and conditions.

The immigration information post by website user , MyTend.com not guarantee correctness

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