By Elizabeth Piper
LONDON (Reuters) - Senior ministers called on Sunday for a truce in a heated debate on Britain's membership of the European Union, saying the ruling Conservative Party must be civil if it wants to stay in power.
The party is deeply split over whether Britain should vote to stay in the EU at a referendum on June 23, and several allies of Prime Minister David Cameron, who supports EU membership, are campaigning to leave the 28-member bloc.
Some lawmakers have criticised Cameron for accusing London Mayor Boris Johnson, a leading Conservative, of putting his own ambition above his beliefs by backing the "Out" campaign.
They said the thinly veiled attack last week on Johnson, the most prominent party member in the "Out" camp, violated an unwritten rule that both sides respect each others' opinions.
"There are different opinions in the party. Everyone has to go out of their way to make sure that the debate within the Conservative Party is a constructive one, an amicable one and one that doesn't involve attacks on each other," said Christopher Grayling, leader of the House of Commons (the lower house of parliament) who supports leaving the EU.
"And then from a Conservative point of view after the event we've really got to make sure that when the result comes in, we come together, we heal the party, we move beyond the discussions at the moment and we win in 2020," he told Sky News, referring to the year when the next parliamentary election is expected.
Cameron has allowed his party members to campaign according to their conscience, saying he cannot force lawmakers to back his campaign in support of EU membership at a referendum which will help shape the future of the bloc - Britain is the EU's second-largest economy and one of its two main military powers.
Many of Britain's leading businesses have said they want to remain in the European Union and the opposition Labour Party has said it will campaign to stay in, overcoming the euroscepticism of its leader, veteran anti-war campaigner Jeremy Corbyn.
Jonathan Ashworth, Labour's shadow minister without portfolio, said: "The squabbling amongst the Conservative Party says more about internal positioning ahead of a leadership contest than it does about what's in the long-term interests of the country."
Cameron has said he will not resign if Britain votes to leave the EU. But the Sunday Times newspaper quoted sources as saying that Conservative lawmakers would mount a leadership challenge against Cameron if he failed to halt so-called 'blue-on-blue' attacks - Conservatives attacking Conservatives.
Former Conservative leader Iain Duncan-Smith, secretary of state for work and pensions, called on his fellow Conservatives to respect each other.
"The general view at the time and I think it should remain now is that don't play the person, play the ball," he told The Andrew Marr Show on the BBC. "We should take a deep breath ... This is about Britain, it's not about us."
(Reporting by Elizabeth Piper; Editing by Andrew Heavens)