David Cameron will make the case for staying in the European Union to Parliament on Monday after London Mayor Boris Johnson became the highest-profile figure to say he’ll campaign for Britain to quit the 28-nation bloc, sending the pound plummeting.

Cameron is due to address lawmakers in the House of Commons at 3:30 p.m. on the reform deal he struck with fellow European leaders on Friday. The agreement is aimed at helping him to argue that Britain is better off in the EU going into an in-out referendum he’s called for June 23.

That stance suffered a setback after Johnson, one of the U.K.’s most popular politicians and prominent Conservatives, said he’ll campaign for so-called Brexit, putting himself in direct opposition to Cameron, his party leader. The pound fell the most since the financial crisis against the dollar as investors woke up to the likelihood of an energized “Out” campaign.

Scotland Risk

“He is a superb campaigner, so he’s a good asset to the cause,” Nigel Lawson, a Conservative former chancellor of the exchequer who leads the Vote Leave movement, said in an interview on BBC Radio’s “Today” program. “What this is about is a declaration of independence by the United Kingdom. It’s about time we became a self-governing, independent democracy.”

U.K. assets are under pressure amid uncertainty over the likelihood of an unprecedented exit from the world’s biggest trading bloc. With polls disagreeing on the outcome of the referendum, concerns are also resurfacing over the possibility of Scotland’s secession from the U.K. in the event of vote to leave.

The pound was down 2.1 percent to $1.4105 as of 12:16 p.m. in London, set for the biggest decline since March 2009. Sterling climbed 0.5 percent Friday, the most in more than two weeks, after Cameron secured the EU deal.

Costing Business

“The dramatic fall in sterling today is driven by fear of Brexit and means that the threat of leaving is already costing British business,” Susan Kramer, economic spokeswoman for Cameron’s former coalition partners, the Liberal Democrats, said in a statement. “Imagine how much worse it will for British exporters if we withdraw from the world’s biggest single market.”

Britain’s Institute of Directors said the deal was sufficient for more than 600 of its members to vote to stay in the bloc. The EEF, which represents engineering, manufacturing and industry firms, said separately that 61 percent its members want to remain in the EU, mainly because membership makes it easier for companies to export.

“Companies, particularly those interested in exporting, do not see the point of the U.K. cutting itself off from its major market,” EEF Chief Executive Officer Terry Scuoler said in a statement.

The view from the business community contrasted with that of Johnson, who told reporters outside his home in London on Sunday that he’d vote to leave the EU “because I want a better deal for the people of this country: to save them money and take back control.”

‘Pop-Star Charisma’

His move was a second blow to Cameron after Justice Secretary Michael Gove, one of his closest political friends, announced on Saturday that he too will campaign for an exit. While Gove, a former Times newspaper columnist, is an accomplished debater, Johnson, known almost universally simply as Boris, is a politician with star power, the author of bestselling books and a television performer. Cameron had urged Johnson to stay on his side.

"He definitely has pop-star charisma and appeal,” Jacob Nell, a U.K. economist at Morgan Stanley, told journalists at a briefing in London. "If the ‘Out’ campaign get it right, they could appeal to both the anti-immigration and the deregulation group of voters and Johnson could help make voting for Brexit seem socially acceptable."

Johnson is among the favorites to succeed Cameron, who’s said he’ll step down as prime minister before the 2020 general election. It means much of the coming debate will be conducted, in public, between members of the governing Conservative Party.

‘Feel Good’

“There’s enormous numbers of voters who are not sure what to do in this referendum,” Johnson’s biographer, Andrew Gimson, said in an interview. “Whether this will help them to make up their minds, I don’t know. But for people who want to feel good about voting to leave, Boris is good at making people feel good about things.”

A telephone poll carried out by Survation on Saturday showed 48 percent of respondents want the U.K. to stay in the EU, compared with 33 percent who would vote to leave and 19 percent who are undecided. In recent months, most phone surveys have shown double-digit leads for remaining in the EU, while online polls have had the race much closer, some suggesting the “Leave” camp is ahead.

No Going Back

Cameron’s spokeswoman, Helen Bower, said there is no going back if Britain votes to leave the EU.

"The process is set out very clearly in the treaties, a vote to leave is a vote to leave,” she told reporters in London. “If the British people vote to leave the government will clearly respect the outcome of that and launch the process.”

Bookmakers responded to Johnson’s announcement by cutting their odds of a vote to leave. Ladbrokes Plc installed Johnson as the favorite to replace Cameron, in front of Chancellor of the Exchequer George Osborne.

"To say that this is a careerist sort of move would be a total travesty,” Johnson’s pro-EU father, Stanley Johnson, himself a former European lawmaker, told the “Today” program. “I can’t think of any more career-ending move than to do what he did yesterday, in the sense that he’s leaving the mayoralty in May. If he wanted to get a nice job in the cabinet on May 8, this is certainly not the way to do it.”