It says on the contract, but is it really legal? I'm honestly asking as I mentioned, I know portuguese labour law quite well (my cousin is a lawyer specialising in labour law and as such, she looks over all my contracts and say what is valid or not.) Have you talked with a lawyer about it over there in the UK? I know the UK didn't sign some EU directives about overtime pay for IT workers so they don't fall on the common EU law, but when I was in England, noone I worked with actually knew if the 'outside work' clause was legal. I personally did freelance work (which my leads knew about) and never got any problem for it even though the contract specified I couldn't.

BTW Professionals are exempt from a lot of the Directives on working time and 99% of IT workers are considered professionals.

Um IANAL but I am an "approved person" as it applies in the UK (ie I could represent some one at a discipline or grievance hearing)

In the UK and the USA employment law descends from the Masters and servants act.

The argument used would be having another job without your employers agreement is a fundamental breach of trust between the employer (master ) and the employee (servant)