Extra armed police are guarding the building and an inquiry is under way to find out how it happened.

MPs say it is possible that the protesters - who support hunting - were helped by someone who works at the Commons in staging their stunt.

MPs backed a ban on hunting in England and Wales by 339 to 155 votes.

The ban will now be debated in the House of Lords.

They have previously blocked a ban but it's possible that the government will use a special power called the Parliament Act to make hunting illegal.

This would mean that the Lords' approval is not necessary.

However, it's likely that the ban would not come in until 2006, to allow people involved with the hunt to find new employment and to re-home the dogs they use.