Raed Salah, the leader of the Islamic Movement in Israel, has been detained by the UK Border Agency.
Arrangements are now being made to remove him from the country.
Theresa May said: ‘We do not normally comment on individual cases but in this case I think it is important to do so.
‘I can confirm he was excluded and that he managed to enter the UK. He has now been detained and the UK Border Agency is now making arrangements to remove him.
‘A full investigation is now taking place into how he was able to enter.’