A major review of Westminster security is underway after five pro-hunt protesters stormed the Commons chamber.

Wednesday's breach was the worst in living memory and the third lapse in security this year.

The eight men were in possession of a fake letter, purportedly signed by two MPs, which confirmed they were visiting in the capacity of electrical contractors.

In an emergency Commons statement Michael Martin said a passholder "apparently gave assistance to the intruders".

The attack has raised fresh questions about the role of the serjeant at arms and the other "men in tights" who guard the Commons chamber.

It came as police and pro-hunt protesters jostled in a series of angry exchanges in Parliament Square.

Security barriers were thrown, smoke bombs were let off and batons were drawn as the protesters outside made a last stand against a hunt ban.

Inside up to 10 protesters, who had given the BBC prior warning of their attack, attempted to storm the Commons chamber during the hunting debate.

Some protesters were apprehended before they reached the Commons, although the others made their way into the chamber from doors leading from the "no" division lobby.

One ran towards rural affairs minister Alun Michael but was dragged to the ground by security officials who ran into the chamber.

It is thought that the protesters posed as building contractors to secure access to areas of the palace not open to members of the public.

Once inside they discarded luminous yellow jackets and made their way down a secret staircase into the area next to the Commons chamber.