Intelligence chiefs are calling for the most sweeping security shake-up in the history of Westminster to prevent terrorists striking at the heart of central London.
Amid fears that government departments and tourist landmarks such as Big Ben are under threat, intelligence experts want to set up a "sterile security zone" around a large swath of London, blocking off key roads and sealing off the Commons debating chamber with bomb-proof screens.
Police and anti-terror chiefs want to close roads around key ministries, including the Department of Health, Foreign and Commonwealth Office and Ministry of Defence as well as Parliament itself, which could extend from Trafalgar Square to Millbank, where MI5's headquarters are situated.
The plan, described as "the nightmare scenario" by one insider, would close several major roads around Parliament to create the security zone at the heart of London's tourist area.
Road closures have already been raised with politicians by anti-terrorism officers and MI5, who are known to be concerned that the Commons could be vulnerable to attack from a truck packed with explosives. Big Ben is thought to be at risk because it is close to a major road junction.
According to a source involved in discussions over the measures, it would be a far more dramatic and visible "ring of steel" than the one imposed on London's financial district at the height of the IRA's bombing campaign on the City.
An interim report by anti-terrorist experts into security at Westminster will be presented to the Leader of the House, Peter Hain, and Commons Speaker Michael Martin this week and is likely to recommend specific security measures for the chamber itself. These are likely to include a plan to seal off the chamber in a bubble of screens.
But a further report is being prepared for the autumn, which ministers believe will recommend expensive road closures around Parliament Square and Whitehall. The zone would not necessarily be closed to pedestrians or vehicles such as buses and taxis, but anyone coming and going in the zone would be liable to checks. MPs were hoping for a more limited pedestrianization scheme, along the lines of a project already being considered by Transport for London, which will close just one side of Parliament Square to traffic.
The interim report into security at Westminster was commissioned before the Fathers 4 Justice purple flour bomb attack on Tony Blair last month. It is likely to be critical of staff at the Commons, who allowed MPs to leave the chamber after the attack. There are serious concerns that, in the event of an attack using a biological chemical agent, MPs would have been in contact with the public.
Westminster sources said there are also likely to be recommendations for further screens at the Commons chamber to create a secure zone for MPs and proposals to beef up the security at the various entrances to the Palace of Westminster.
Hain is known to be concerned that extensive road closures could run counter to his plans to engage the public in the political process by opening up Westminster further. But ministers know they will be exposed to charges of complacency if they reject recommendations from the police and security services.
Simon Hughes, Liberal Democrat spokesman for London, said the proposals were not justified: "I understand the pressure to take all possible steps to protect government offices, Parliament and high-ranking figures from terrorism. But any general or regular closure of streets in and around Westminster sends completely the wrong signals and, at this stage, would be an overreaction. These sorts of restrictions can only be justified in exceptional cases and for the shortest period necessary and that's not a situation we're in."