David Cameron's pledge to leave a centre-right grouping in the Brussels parliament is his "Achilles heel", Geoff Hoon has said.

The Europe minister's comments came amid fresh revelations of turmoil in the Conservatives over moves to leave the European People's Party bloc.

Cameron's commitment to leave the EPP has bitterly divided his MEPs and threatened to isolate him from other European leaders.

Attempts to form a new anti-federalist group have also foundered on divisions between potential Czech and Polish partners.

Meanwhile, a leaked email written by Desmond Swayne, the Tory leader's parliamentary private secretary, warned the policy would be "blown off course".

And it added that some MPs thought shadow foreign secretary William Hague, tasked with implementing the withdrawal from the EPP, had "wobbled".

Hoon said the Conservative policy is in "such a mess that both pro-European and anti-European Tory MEPs are threatening to quit the Conservative group in the European parliament".

"This major commitment David Cameron gave during the Tory leadership election is now crumbling around his ears," he said.

"Fulfilling the pledge would leave the Tories totally marginalised, allied with right-wing parties outside the mainstream, and fudging the promise would undermine the credibility of any future policy commitments.

"The Tories are as divided as ever over Europe."

The Europe minister added that Cameron's EPP pledge "has become his Achilles heel".