Mr Blair was speaking during his weekly half-hour appearance in the Commons when a projectile filled with purple powder hit him on his back. MPs were immediately evacuated. Early tests suggest the powder was "benign" and not dangerous.

Fathers 4 Justice claimed responsibility for the incident. Scotland Yard confirmed that two men had been arrested.

The incident comes just weeks after a £600,000 temporary security screen was installed in the Commons.

It appears the projectile may have been thrown from an area of the gallery reserved for MPs' guests and notable visitors.

Deputy Prime Minister John Prescott and Chancellor Gordon Brown, who were sitting either side of Mr Blair, both looked up at the public gallery as the dust descended at 12.18 BST.

MPs were seen wafting something away from themselves as the House was suspended.

A spokesman for Fathers 4 Justice said two members of the group were involved in the incident to highlight MPs' failure to help fathers gain access to children through the courts.

Labour MP Kevin Brennan said a stain was "clearly visible" on the back of Mr Blair's jacket as the dust hit him.

Tory ex-minister Michael Portillo said he thought "one dose" of the dust had been thrown "before the prime minister reacted".

"The prime minister only appeared to look round to his side at the second dose," he said.

"I thought it was likely to stain his coat or something, but as far as I could see, it was actually falling behind him."

Commons leader Peter Hain said: "This was a serious incident. I have asked for an immediate report on the circumstances and what additional security provisions may be necessary."

Liberal Democrat MP Mark Oaten, who was in the chamber during question time, said he heard a large shout from the public gallery area from an individual who appeared to be "running forward".

"He threw what looked like a paint ball at the prime minister and shouted Fathers 4 Justice," he said.