Its manifesto for next month's European Parliament elections will pledge that a Tory government would negotiate a "flexibility clause", allowing all member states to choose which new EU laws to adopt.
Downing Street attacked it last night as a "pick and mix" plan that would lead to a "half-in, half-out relationship with Europe".
But the Tories denied that they were trying to unravel the EU, saying the clause would not apply to "core policies" such as the single market, free trade and competition. New applicants would, however, be allowed to opt out of some existing EU policies for ever, making it easier for the EU to expand into Eastern Europe.
Speaking in Hungary last night, William Hague said that both existing and new members should be "free to develop a mix and match approach".
He said the EU's governing treaties should be amended to "allow countries not to participate in new legislative actions at European level which they felt they wished to handle at a national level".
Mr Hague said: "In a restaurant every diner is not required to order the same food from the menu, nor should members of the EU be forced to sign up to every policy coming out of Brussels."
Calling for a flexible rather than federal Europe, Mr Hague said the EU was "taking yet more powers from the nation states to the centre ... The truth is that, in order to accommodate states with widely differing conditions, the EU must be prepared to devolve power to a lower level.
"As the future British prime minister, I will always stand up for Britain's interests in Europe, even if that means being in a minority of one."
Mr Hague also said "fighting to save the pound" would be a key part of the Tory campaign until the next general election.Reuse content