Coalition on Euro collision course
Liberal Democrat Cabinet minister Danny Alexander risked inflaming tensions with the Tories over Europe by branding Eurosceptics "enemies of growth".
Despite chaos in the single currency zone threatening to derail the world economy, Mr Alexander insisted sharing sovereignty made Britain stronger.
The intervention came as the Lib Dem leadership sought to soothe anxious activists at the party's conference in Birmingham by stressing differences with their coalition partners.
Deputy Prime Minister Nick Clegg used an interview to make clear he would not bow to Conservative demands for the 50p tax rate to be scrapped unless there were major concessions in return.
Mr Clegg claimed his ministers were imposing their will on their "political enemies" in government, while there have been a series of risqué jokes from the platform at the expense of senior Tories.
In his speech to members, Mr Alexander insisted that coalition ministers were "united in pursuing a policy of pragmatic, practical engagement in the EU".
But he rounded on those who have been demanding a loosening of ties with the struggling union - including many Tory right-wingers. "Sadly, Eurosceptics on left and right fail to understand Winston Churchill's central insight that sharing sovereignty strengthens influence and isolation weakens us," Mr Alexander said.
"Scottish Nationalists make the same mistake. We will never let the anti-Europeans or nationalists frustrate our national interest. They are enemies of growth."
Deepening, strengthening and deregulating the single market was a central aim of Britain's European policy because it would bring jobs and growth, he added.
The future of the 50p top tax rate has become a source of friction in the coalition amid calls from senior Tories such as George Osborne and Eric Pickles for it to be abandoned. Critics believe the levy is raising little or no revenue, and makes the UK less attractive to entrepreneurs. The Treasury is not expected to publish figures on its effectiveness until next spring.