Prime Minister David Cameron has accused Boris Johnson of “getting it wrong” on the EU and said he hoped for a “civilised and reasonable” debate.
Cameron said he had “great respect” for the London mayor and that he had a “very strong future in British politics” but added “on this issue I think he has got it wrong”.
The premier admitted he was “disappointed” Johnson was campaigning for the leave campaign after exchanging many texts and e-mails with him over the issue.
It comes after he launched a stinging attack on Johnson in the House of Commons calling the mayor’s suggestion of a second referendum on the UK’s membership of the EU “for the birds”.
Cameron was speaking as he made his case to remain in the EU to voters for the first time at an event at a Slough business.
Earlier, Johnson denied the EU would split the Conservative Party and that the Tories would unite behind Cameron after the EU referendum, despite signs of a deepening rift in the party over Europe.
Former foreign secretary Lord Hague warned the party it could lose the next election if it became embroiled in in-fighting over the EU.
Speaking yesterday morning, Johnson said: “I think one of the big differences between now and what I remember back in the 1990s is that the Conservative Party actually has a much better team spirit, much better feeling about this stuff.
“I’m sure, I have absolutely no doubt that after this is over the Tory Party is going to unite again around David Cameron’s leadership.”
Cameron told people at the Slough event that if the UK hadn’t been in the EU then one of the 21/7 terror attack plotters would not have been captured and jailed. He said the UK was safer to remain in.
And he said that businesses were better off in the European Union, highlighting the support from business leaders in a letter published yesterday morning.
Johnson sharply  criticised the letter signed by 200 top business leaders warning leaving the EU would “deter investment and threaten jobs”.
Some 198 business leaders including Roger Carr, chairman of BAE Systems, BP CEO Bob Dudley and Ron Dennis, chief of F1 team McLaren, wrote a joint letter published in the Times, backing Prime Minister David Cameron’s deal to reform the EU.
“Following the prime minister’s renegotiation, we believe that Britain is better off staying in a reformed European Union,” they wrote, adding Cameron had secured important commitments on improving competitiveness within the bloc.
“We believe that leaving the EU would deter investment, threaten jobs and put the economy at risk,” wrote the business chiefs, who between them employ around 1.2mn people.
“Britain will be stronger, safer and better off remaining a member of the EU,” it concluded.
Separately, Nissan, which produced 450,000 vehicles in Britain last year, issued a statement saying it “made the most sense for jobs, trade and costs” to stay in the EU.
“For us, a position of stability is more positive than a collection of unknowns,” said the statement. “However, this is ultimately a matter for the British people.”
Chairmen or chief executives of 36 companies from key share index FTSE 100, including national giants such as BT, Marks & Spencer, EasyJet, Burberry and Vodafone, all signed the letter.
Two leaders of US firm Goldman Sachs in Europe were also signatories.
However, critics pointed out that many large employers such as retailers Tesco and Sainsbury’s and banks RBS and Barclays had not signed the letter and accused Cameron of “bullying” businesses into supporting his position.
“The truth is that despite the bullying of a prime minister who has no real business experience, it is other normal commercial factors which will determine the continued success of British businesses to invest and grow,” said Richard Tice, co-founder of pro-Brexit group Leave.EU.
Experts also pointed out that smaller businesses were less pro-EU than their larger counterparts.