The security services are heading a review of anti-terror measures at the Commons after purple flour was thrown at Tony Blair as he addressed MPs.
Mr Blair was replying to the Tory leader when the dust was thrown
Commons leader Peter Hain told the BBC Wednesday's protest was a "wake-up call" and a "very embarrassing lapse".
Mr Hain said the attack could have "been ricin or anthrax ... and killed... large numbers of MPs".
After a meeting with security service chiefs, he said: "They are advising on what action needs to be taken."
Wednesday's alert during the prime minister's weekly question and answer session has given a fresh urgency to the £5m upgrade of security at Westminster already underway.
Mr Blair was hit on his back by the harmless substance - flour dyed purple - prompting an evacuation by MPs.
Campaign group Fathers 4 Justice claimed responsibility. Two men aged 50 and 36 have been arrested.
The incident happened despite a £600,000 bullet-proof security screen being installed in the chamber at Easter.
New security measures are being introduced, including a new entrance to the Palace of Westminster.
On Thursday, Mr Hain said recommendations from the security services will be considered next week and "some hard lessons" needed to be learnt from the flour bomb attack.
Protester Ron Davis
But he insisted: "We need to strike a proper balance between proper security and the rights of public access to Parliament and its members."
He told the BBC: "It underlines the point that I argued in the Commons last month, when moving a motion to erect a security screen against this kind of attack from the public gallery ... that we needed to improve our security radically."
He added that he had been warned of the threat of such an attack from terrorists by the security services last year.
But he had faced criticism of moves to install a glass screen in front of the seats members of the public sit in inside the Commons chamber, he said.
"Frankly there has been a very old-fashioned culture around the House of Commons for far too long, not just from many of the authorities involved, but also from MPs," he said.
"Everybody wants to be in touch with voters - voters should still be allowed to come into Parliament and guests allowed - but we are living in a very different world, a world of suicide terrorists, not a world in which these things didn't exist."
Home Secretary David Blunkett said the security services would bring forward recommendations "very quickly indeed", but it could mean there would be more restrictions on the public areas of the House of Commons, he said.
Proceedings were suspended for more than an hour following the incident at 1218 BST.
It is believed the suspects got tickets to the Commons at a charity auction.
Father-of-two Ron Davis, from Worthing, West Sussex, threw a flour-filled condom from part of the gallery reserved for MPs' and Peers' guests and notable visitors as a second activist, named by police sources as Guy Harrison, held up a poster.
A spokesman for Fathers 4 Justice said Mr Harrison was from Steyning, West Sussex.
In a statement, Labour peer Baroness Golding said the two protesters were guests of hers and she offered "unreserved apologies" to Commons Speaker Michael Martin, MPs and fellow peers.
Matt O'Connor, spokesman for Fathers 4 Justice, said two members of the group wanted to highlight MPs' failure to help fathers gain access to children through the courts.
Mark Oaten, the Liberal Democrats home affairs spokesman, called for an urgent review of security arrangements.
For the Conservatives David Davis described the incident as "profoundly disturbing in the current security situation."
HOUSE OF COMMONS POWDER ATTACK
1: Projectiles thrown by two protesters in section of gallery above Commons reserved for ambassadors, members of the House of Lords, their guests and MPs' guests.
2: Purple powder sprays over Labour backbenchers on benches below.
3: Two projectiles reach Tony Blair, standing at the government despatch box. One hits his back, the other hits the floor at his feet.
4: Speaker Michael Martin suspends proceedings.