A full review of security at the House of Commons has been ordered after Tony Blair was hit by a purple flour bomb hurled by a protester during Prime Minister's Questions today.

Fathers 4 Justice, a group campaigning for greater access to children, staged the demonstration in which one man threw a flour-filled condom from the public gallery as a second, unnamed, activist held up a poster. Two men, aged 50 and 36, have been arrested.

Michael Martin, the Speaker, immediately suspended the sitting halfway through Mr Blair's weekly question and answer session. Mr Blair was unhurt, but the back of his suit was splattered with the flour.

David Blunkett, the Home Secretary, said that the incident would mean "even greater restrictions" on public access to the Commons.

It comes as a huge embarrassment for Parliament's security staff as a massive security screen was recently installed in the public gallery to prevent just such an incident.

But it does not cover the front three rows of the gallery reserved for peers and their guests.

The security screen was erected, against the views of some MPs, after advice from the Director of the Security Service last summer.

Guests of Members and peers were allowed to sit outside the screen on condition that they were personally accounted for.

The protesters were signed in by Baroness Golding, the Labour peer, who said apologised to MPs this afternoon for the "very serious incident".

The former MP said in a statement: "I have spoken to the Speaker offering my unreserved apologies to him and through him to my former colleagues in the Commons.

"I shall, of course, give every support and co-operation to the authorities who are now investigating.

The powder hit not only Mr Blair, but appeared to splatter around him and his colleagues on the front bench, John Prescott, the Deputy Prime Minister and Gordon Brown, the Chancellor.