Boris Johnson Is 'Wrong' And Should Expect 'Passionate' Brexit Debate, Says David Cameron

David Cameron has said he wants a "civilized" argument with Boris Johnson over Brexit, the day after he ridiculed the London mayor in the House of Commons.

The prime minister said today he believed Boris has a "very strong future in British politics", but attacked his Tory rival for being "wrong" for believing Britain should leave the European Union.

"I have huge respect for Boris as a politician. he is a great friend of mine. He is a fantastic mayor of London. He has a lot to give to the Conservative Party. He has a lot to give to this country. But on this issue I think he has got it wrong and I think he has reached the wrong conclusion," Cameron said.

Cameron said he expected a "a strong and passionate debate" between the pair over the course of the referendum campaign but hoped it would be "very reasonable and civilised".

Yesterday, Cameron used his statement to MPs on the referendum to brutally dismiss Boris' arguments for Brexit. Eurosceptic MPs later asked the prime minister to "be kind to Boris" during the campaign.

Speaking outside his London home this morning, Boris insisted there was "no doubt" that the Conservative Party would "unite again around David Cameron's leadership" once the referendum was over.

Former Conservative leader and foreign secretary William Hague warned today the party must avoid "a sustained battle" over the EU during the referendum that would "take a generation to heal".

Speaking to 02 workers in Slough today, Cameron rammed home the message that being prime minister had made him more pro-European.

And he repeated the claim he has no ulterior motive driving his position - a phrase seen as a dig at Boris' Tory leadership ambitions.

"I'm saying this because I feel this so strongly. I am not standing again as prime minister and I want people to know that I'm speaking about this issue after thinking about it very very deeply ," he said.

"I think six, ten, 15 years ago, I don't think I believed that Europe was as important to our security as I believe it is today, because I've seen with my own eyes just how important the security and intelligence sharing of information is."

"I'm not sure six, maybe 10 years ago, I thought Europe was so important for Britain getting things done in the world."

Cameron's claim Britain is more secure as a member of the EU was a direct rejection of work and pensions secretary Iain Duncan Smith's claim the country is vulnerable to a Paris-style attack.