Plans to install a £1.3m bullet proof screen in the House of Commons have been approved by MPs.
Concrete blocks already encircle the Houses of Parliament
The screen will seal off the public gallery from the main debating chamber.
A temporary barrier, costing £600,000, was erected during the Easter break and is designed to protect MPs and ministers from possible terror attacks.
Commons leader Peter Hain revealed that the move had been taken after advice from security service MI5 about a possible biological attack.
Mr Hain said the MI5 director general Eliza
Manningham-Buller had acted on "clear intelligence" when she made the "unequivocal
recommendation" that a screen be installed.
The minister graphically spelt out how al-Qaeda terrorists could throw a phial of anthrax or ricin into the Commons if there was not a screen.
"The particles would immediately begin spreading throughout the chamber -
because of the way the air flows work, within minutes total contamination would
occur," he said.
A comprehensive review of security at Parliament is under way. Fears were raised when two Greenpeace protesters breached security last month to climb Big Ben during anti-war protests.
Some MPs argue the security screen sends out the wrong signals and is a barrier between the politicians and the public who elects them.
Labour's Sion Simon said: "It says to Mr Terrorist, 'I'm frightened of you, you
have made me change my ways, I have put a barrier between my Parliament and my
MPs voted by 112 votes to 76 - a majority of 36 - to go ahead with plans for a permanent screen. It will be installed during the summer of 2005.