Yes, officially you are due to pay tax on the prize or any other income you receive from your employer. However at my previous company, where we used to give cash awards for special performance, or for long service awards for example, the company would account for these sums annually to the Inland Revenue so that the employee wasn't penalised. If this was a work competition to come up with new ideas for efficiency or productivity or for good attendance/performance I think it would be worth asking your employer if they would pay the tax. If it was just a "fun" competition, then I think you should accept that it's reasonable to pay tax on it.