The ruling Labour Party is trailing the opposition Conservatives in the opinion polls ahead of an election expected on May 6, and a strike that would disrupt Easter holiday plans for tens of thousands of people is the last thing Brown needs.

He has called the plan "unjustified and deplorable,” urging BA and Unite to resume talks.

Unite is Labour's main financial backer, and the Conservatives have seized on the threatened strike to mount a full-scale attack on what they present as left-wing union bosses' stranglehold on a party dependent on their cash.

"BA needs this strike like a hole in the head," Theresa Villiers, the Conservative transport chief, told a news conference, calling it "damaging and unnecessary.”

"Labour's paymasters at Unite are determined to wind the clock back to the 1970s, when strikes brought this country to a standstill," she said.

Most BA cabin crew plan a 3-day strike starting on Saturday, followed by a 4-day walkout from March 27.

Unite's decision to announce these plans resulted in BA removing a formal offer on staffing levels made to employees, as it was conditional on the union not naming any strike dates.

A Unite boss said on Tuesday the company should bring back the offer it had withdrawn.

"What we need to do is put the offer on the table, let's get 100 percent of these flights flying and get serious negotiations off the ground again and I make that offer publicly," the union's Joint General Secretary Tony Woodley told BBC Radio 4.

Regardless of whether or not talks resume between BA and Unite, the Conservatives appeared determine to broaden out the issue of the strike into a vitriolic attack on Labour's links with unions.

They launched a billboard campaign entitled "Cash Gordon,” featuring a picture of the prime minister clutching a fistful of bank notes behind his back.

"Unite are funding Brown's election campaign. What will he give them in return?" the billboards read.

The Conservatives have faced uncomfortable questions of their own funding since one of their biggest donors, millionaire businessman Michael Ashcroft, revealed on March 1 he was not based in Britain for tax purposes.