The planned replacement of Westminster's historic iron railings by a forbidding 15ft-high barrier topped with razor wire is stark evidence of the security threat now facing Britain.
Tougher measures were already on the way to protect Premier Tony Blair and MPs.
Now the embarrassing scaling of Big Ben by two anti-war pro- testers last weekend has added urgency to a clampdown dubbed Operation Fortress Commons.
A minister close to its planning said: "We must start taking security here seriously if we want to avoid a strike at the heart of our democracy."
Police with shoot-to-kill orders will patrol Parliament's perimeter, currently protected from car bombers by concrete blocks.
English Heritage opposes removal of the iron railings but a senior security source said: "A terrorist strike is now inevitable and certain changes must be made."