Radical security changes to protect the Palace of Westminster from terror attacks are being recommended by MI5.
Concrete blocks could be dangerous if blown up
A report by security chiefs is expected to propose erection of a steel barrier to protect Parliament.
It is feared current concrete barriers could be dangerous if blown up - while the Big Ben clock tower could fall on the Commons chamber if targeted.
The review was prompted by a flour attack on Tony Blair as he addressed MPs this summer.
MI5's report into protecting the Palace of Westminster is due to be given to the House of Commons' Commission, the body responsible for overseeing the building's security.
The security service is urging the replacement of large concrete blocks which ring Parliament where it comes closest to public roads.
Experts fear terrorists could turn the blocks into dangerous projectiles by blowing them up. They are urging their replacement with a steel barrier.
Flour bomb: PM targeted from public gallery
The report is also expected to recommend creating a powerful new Parliamentary security director.
In May, the Speaker halted Prime Ministers' Questions when demonstrators threw condoms containing purple flour at the government front bench.
The demonstration came in the middle of £5m of continuing security improvements to Westminster, including a bullet-proof screen which had failed to stop the incident.
MI5 is also reportedly planning a radical change to its intelligence gathering operations with proposals for a regional network of bases.
The Times newspaper reports the home security agency will recruit agents in areas where they are fears of Islamist extremists targeting Muslim communities, principally the West Midlands and North West England.
The plan to shift intelligence gathering beyond London is also aimed at improving ties with regional police Special Branch operations, the Times reports.
MI5, which has had a succession of budget increases, expects to take its total staff up to 3,000 within four years.
It is also continuing efforts to recruit from within British Muslim communities - including via Arabic pages on its official website.
Ken Macdonald QC, the Director of Public Prosecutions (DPP), is also seeking the extension of powers to help bring successful terrorist trials.
A spokesman for the Crown Prosecution Service said Mr Macdonald wanted an increase in the time allowed to hold a charged defendant before having to go to full trial.
Defendants held in custody must normally be bailed if the case against them is not ready for trial 112 days after first being sent to crown court.
But Mr Macdonald, the DPP since August 2003, wants to change this time limit, saying investigators need longer to build effective prosecutions because terrorism cases can be so complex. In May he said
"These cases need more time for investigation," said the spokesman. "Sometimes you may need evidence from abroad and the time constraints may need to be more liberal."
The spokesman added the DPP was also looking into other terrorism-related trial powers.
These could include powers to question people under compulsion, authority for plea-bargain deals or immunity from prosecution for key informers.
"In current times the fight against terrorist is more complex," said the spokesman.
"Immunities from prosecution [for informers helping] in the case of some very dangerous individuals involved in very serious crimes may be something which could be considered."