Mr Hague told a gathering of Conservative MEP candidates that none of them would be allowed to run individual campaigns or say "whatever they like" in the run up to the poll next June.
He said that dissent could ruin the great opportunity offered by the European Parliament elections to prove that the Tories were back in business.
Mr Hague issued his call for unity when he outlined the Conservatives' Statement of Principles on Europe at a conference in Leicestershire.
The statement, which will form a basis for the party's election manifesto next year, maps out a vision of a low tax, deregulated European Union which embraces new democracies in Central and Eastern Europe.
It urges co-operation, but not integration, between states on areas such as defence and the fight against crime, reform of the European Court of Justice and a strengthening of the role of MPs in scrutinising EU draft legislation. However, the manifesto's strong message would be obscured if candidates opposed the official policy of ruling out a single currency for two Parliaments, he said. "No one can afford to run an individual campaign ... We must speak with one voice."
Although the party's Euro-sceptic stance remains, Mr Hague declared that Conservatives would never be "Little Englanders" and claimed that his was the only "true pro-European party" in the UK.
He said that Europe's 20 million unemployed were a "political time-bomb" and stressed that lower social and business costs should be a top priority.
The Tory leader said that the EU should set itself the goal of global free trade by 2020, a deadline that could be achieved by the creation of a new transatlantic free trade area between Europe and North America.
Instead of fighting against the United States, Canada and Mexico, the EU should work with their North American Free Trade Association, NAFTA, to cut tariffs, boost trade and jobs.