A Home Office spokeswoman insisted the announcement, revealing 300 new police officers and staff will be put on anti-terror duties, had been given "careful consideration", including discussion with the Cabinet Office.

But it is understood the Cabinet Office had not completed its assessment of whether Ms Smith could go ahead during the period of silence – "purdah" – in the weeks before a vote.

Tories lodged an official complaint, saying Ms Smith had broken purdah rules just two weeks before polling day. The system is designed to prevent the party in power having an unfair advantage in an election campaign.

Some Whitehall officials privately expressed surprise at Ms Smith's decision to make the high-profile announcement on morning TV and radio shows, followed by a speech to police.

Shadow Communities and Local Government Secretary Eric Pickles, who reported Ms Smith to the Cabinet Secretary, Sir Gus O'Donnell, said: "This was quite clearly a carefully choreographed piece of political spin to cover up the fact that Labour have cut the number of police on the streets. We are seeing once again a desperate attempt by Gordon Brown to get back on the front foot in a week when the news agenda has been dominated by rumours of splits and divisions in the Government."

He added: "It is clear that Labour Ministers have intentionally broken Cabinet Office rules in an attempt to create a political smokescreen."

A Home Office spokeswoman said: "The Home Office attaches great importance to adherence to the rules governing elections and gave this issue careful consideration, including discussing it with the Cabinet Office.

"The Home Secretary's speech today is about the prevention of terrorism, an issue of fundamental national importance and on which the Government has a duty to act. It is on this basis that the Home Secretary made her speech.

"As she made clear in her radio interview this morning, she believes that this is an issue which transcends party lines and is a response to a serious and urgent threat to our national security. The Home Office took the view that it was appropriate that an announcement on an issue of national importance should be made."

Electors go the polls in some English and Welsh councils, as well as for the London mayor and assembly, on May 1.

A Tory spokesman rebutted the Home Office's claim that the issue transcended party politics.

"The issue of police officers is of particular political sensitivity, given police authorities across the country are now cutting the number of police officers and hiking the police levy on council tax bills by way above inflation."