THE Labour hierarchy is appealing to the party's tribal instincts in relation to the AV referendum, stressing how a Yes vote would deliver a powerful blow to David Cameron's premiership.

Sources close to Ed Miliband, the Labour leader who is strongly backing the Yes campaign, suggested that defeat for the No camp would lead to senior right-wingers seriously questioning the Prime Minister’s judgment in cosying up to Nick Clegg and the Liberal Democrats.

The Herald has already been told by one senior Conservative minister that a Yes vote next week would be the “most destabilising” result for the Coalition as there would be a major backlash from discontented rightwing Tories.

Yesterday, Lord Mandelson urged Labour supporters to “think strategically” when casting their votes a week tomorrow.

“If David Cameron, having opened the way to this referendum through his negotiations with the Liberal Democrats, were to see a Yes vote his party would never forgive him,” declared the former business secretary.

“He has many critics, many detractors in the Conservative Party, they are desperately searching for some weapon to use against him and to pitch him out.

“If there was a Yes vote that weapon would be placed into the hands of his critics and they would use it,” he added.

Earlier, on a campaign visit to Kent, the Prime Minister again extolled the virtues of the first past the post system, arguing that a shift to AV for Westminster would keep “dead governments on life support machines”.

He told an audience in Maidstone that first past the post was straightforward. “It can be summed up in one sentence: we all vote for our favourite and the one with the most votes wins. Simple. It is a system so democratic that each person’s vote has the same value ... the same worth. You only get to vote once. That’s not the case with AV ...”

His remarks followed the first Cabinet meeting since senior Coalition figures on opposing sides of the AV argument took swipes at each other. Downing Street said there was some “light-hearted banter” but AV was not formally discussed.

At the weekend, Mr Clegg described Mr Cameron’s defence of the current Westminster system of electing MPs as the “death rattle of a right-wing elite ... who want to keep things the way they are”.