Senior Labour MPs demanded yesterday that Cabinet Secretary Jeremy Heywood take action to stop top Tories “electioneering at taxpayer’s expense.”

Shadow cabinet office minister Jon Trickett said the Civil Service chief must put an “immediate halt” to party political campaigning by ministers on government business.

He told the Star: “We’re getting close now to a position where this is a breach of fundamental constitutional principles.”

The call comes after 21 government ministers were dispatched on a taxpayer-funded tour on Thursday — with most swooping on swing seats.

PM David Cameron, Chancellor George Osborne and Deputy PM Nick Clegg all took part.

The Cabinet Office insisted the purpose of the visits was to promote local growth deals, but a template press release issued by Tory HQ and leaked to the BBC urged MPs and candidates to use visits for political advantage.

It instructs local Tories to simply add their name and constituency to a statement saying the growth deals mean “more jobs and security for hardworking people.”

Mr Trickett said the locations for visits — 98 days ahead of the general election — proved that they were politically motivated.

“It’s wrong that ministers should be going on taxpayer-funded political campaigns at any time, especially the election period,” he said.

“It’s equally wrong that taxpayer’s money is being used by the government and directed at marginal seats in an attempt to influence the election.”

And he added: “I think it’s incumbent on the Civil Service to stand up for the principle of neutrality, especially in the election period.”

Asked to respond to the concerns yesterday, a spokeswoman said only that “the announcements were all government policy.”

She also said all regions of England received cash from the government’s £1 billion growth fund.

However most of the 25 constituencies selected for visits by ministers were marginal and could prove crucial in deciding Britain’s most unpredictable election ever.

The Prime Minister visited Exeter, where the Tories are vying to win the seat from Labour, along with Camborne and Redruth, where his party hopes to fight off a Lib Dem challenge.

The Chancellor was in Southampton Test to boast about the local growth deal, which will also benefit the Tory/Labour Southampton Itchen marginal.

And the embattled Deputy PM sped through four south-west seats — all of them held by the Lib Dems.

Former shadow deputy PM and Labour MP for Exeter Ben Bradshaw said Mr Cameron’s official visit to his constituency was in fact “blatant electioneering.”

Speaking to the Star from his constituency yesterday, he said: “Cameron announced nothing new in the south-west, it was just rehashed announcements.

“He came here paid on a taxpayer-funded trip and it was rank electioneering.

“The irony is that this government has cut investment in infrastructure.

“I think people see through this but the head of the Civil Service needs to step in and intervene because it’s completely unacceptable.”