LONDON • Prime Minister David Cameron started selling his deal on Britain's membership in the European Union to voters with a direct appeal to Mr Boris Johnson, the last senior figure in his Conservative Party still to declare which side he will join in the campaign.

"I would say to Boris what I say to everybody else, which is that we will be safer, we'll be stronger, we'll be better off inside the EU," Mr Cameron told BBC Television's The Andrew Marr Show yesterday, the day after announcing that Britain will hold a referendum on EU membership on June 23. "You boil it down to if you love this country, and I love this country so much, and you want what's best for it," he said.

Mr Cameron is struggling to prevent his Conservative government from fracturing, after securing an agreement with fellow EU leaders overhauling the terms of Britain's membership.

Following an emergency Cabinet meeting last Saturday, six ministers said they would defy the Premier and campaign to leave, among them Mr Cameron's long- time friend and ally, Justice Secretary Michael Gove.

All eyes are on which way Mr Johnson will jump. A popular figure with voters, who mostly know him by his first name, Mr Johnson is seen as potentially the most influential Brexit campaigner, should he choose to back the "Out" camp. An Ipsos Mori poll on Feb 17 found that he is second only to Mr Cameron, when it comes to influencing whether voters choose to stay or go.

Mr Johnson, an Oxford University contemporary of Mr Cameron's who is nicknamed "BoJo", is known for his witty soundbites and has a high public profile. He is one of the favourites to succeed Mr Cameron, whose second term in office expires in 2020, and has said he will not serve a third.

Given his popularity, his support for either side may prove key to the performance of the sterling during the campaign, according to Morgan Stanley. Should he choose to back the Brexit campaign, "the pound should come under immediate pressure", analysts led by Mr Hans Redeker wrote in an e-mail report last Friday.

Mr Johnson was expected to make a statement clarifying his position late yesterday London time, according to newspaper reports.

The political and economic stakes for Britain are rising, as campaigning begins for the June referendum. With markets already braced for volatility, Scottish First Minister Nicola Sturgeon upped the ante yesterday, telling the BBC that a vote to leave the EU without Scotland's backing "would trigger a demand for a second Scottish referendum".

Business Secretary Sajid Javid and Home Secretary Theresa May, both seen as wavering over which way to vote, threw their support behind the campaign to remain.

BLOOMBERG, AGENCE FRANCE-PRESSE