Spy chiefs want police to be placed outside polling stations where senior ministers will vote. Schools, community centres and church halls acting as polling stations will be advised to install CCTV and sweep for explosives.

With the two main political parties yesterday trading blows over their tactics on the terror bill, ministers are preparing for an election dominated by security. Peter Hain, leader of the Commons, called Michael Howard an “attack mongrel” and accused the Tory leader of endangering national security for party advantage and of behaving in “his normal shameless way”.

The Tories accused Tony Blair of trying to “spin” the war on terror. David Davis, the shadow home secretary, said that Labour could be vulnerable to criticism if it tried to focus on its security record.

“It is irresponsible, unethical and unwise to try and make security a political issue. It is playing politics with the security of the nation,” said Davis.

The advice to protect polling stations is being prepared by officers from the National Security Advice Centre, an MI5 unit set up last April to look into ways of protecting key government buildings from attack.They have been seconded to a secret Whitehall committee examining the prospects of an election attack.

One senior government official said: “The committee is involved in preparing security advice for returning officers and that advice will be circulated to polling stations.”

With the poll expected on May 5, there is growing concern in Whitehall that terrorists trained by Osama Bin Laden might want to launch an election “spectacular”.

One senior government figure who advises Blair on terrorism warned: “They may time an attack for the election, that is very likely. Look what they did on March 11 last year — they changed the course of the Spanish election.

“What would it matter to them who they kill as long as they did it in the middle of a general election?”

MI5 analysts fear that polling stations in high-profile constituencies could present a “primary” target to terrorists. These include stations in Sedgefield, where Blair is MP; Norwich South, where Charles Clarke, the home secretary, sits; and Blackburn, the constituency of Jack Straw, the foreign secretary.

Security sources point out that as well as being a date when the attention of the world will be fixed on Britain, election day is the one day when senior politicians must be at a specific location to vote.

“Polling stations will be advised that they will need increased vigilance,” said one official. “This will involve obvious security measures such as the presence of a policeman and CCTV coverage.”

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