May: 'Something went wrong' with Salah

According to the Daily Telegraph, Sheikh Salah was able to enter Britain without being stopped because border officials believed he was flying to Terminal One rather than Terminal Five and sent an alert to the wrong place.

But Mrs May said there should not be speculation on what had happened and that a full investigation was taking place.

"There is a normal process that is undertaken by the UK Border Agency in relation to any exclusion orders that I sign," she said. "That process may differ depending on the circumstances of the individual who is being excluded from the United Kingdom. There is a normal process of notification and then obviously of indications to those at ports of entry that this is an individual who should not be allowed to enter the United Kingdom.

"That did not work in this instance. Something went wrong. We are urgently reviewing this to see what it was that went wrong."

She would not be drawn on the reason for the banning but said it was for "unacceptable behaviour".