At least eight Cabinet ministers and 1,000 business leaders could campaign to leave the EU.
In a major boost to the No campaign some of the Conservative Party’s biggest donors have signed up to support efforts in the run-up to a planned 2017 referendum, The Daily Express reported on Monday.
It is understood they have amassed a £7million war chest for the battle to begin next month.
Securing the Tory heavyweights is an open challenge to David Cameron, who has made it clear he will campaign for Britain to stay in the EU.
Cameron has insisted that ministers must support his renegotiation with Brussels.
Work and Pensions Secretary Iain Duncan Smith and Chris Grayling, leader of the House of Commons, are the most likely to quit rather than support membership.
Leaked documents show that the No campaign’s chief executive will be Matthew Elliott, who successfully led the referendum campaign to defeat proposals for an alternative voting system in 2011.
He is bringing with him up to 1,000 business leaders who support Business for Britain, the Eurosceptic pressure group he runs.
Leading donors have been told the first phase of the campaign will focus on the dangers of a Yes vote.
It will argue that Britain’s £10billion net EU contribution would be better spent on the NHS.