After the election of William Hague as Conservative leader, some moderate Conservatives said he had given them strong private assurances that they need not fear a further right-wing lurch against Europe.
But last night's speech by Mr Howard, to New York's Manhattan Institute, will bear out the worst fears of Tory left-wingers, who will publicly defy the new leadership if it insists on pushing such a strongly anti- European line.
Raising the spectre of individual member states being swamped by the federal ambitions of some European leaders, Mr Howard said: "The nation state must remain the basic building block of the European Union.
"People are naturally proud of their own countries. Politicians who ignore that pride and sense of identity do so at their own peril. To undermine institutions and ways of life which have developed over hundreds of years in the pursuit of a federal Europe would be the utmost folly."
On numerous occasions, senior Conservatives like Sir Edward Heath and Lord Howe have pointed out that the EU does not threaten the integrity of the nation state, national pride, or traditional ways of life.
But Mr Howard went on to warn: "Europe is not a nation, and it is dangerous for the EU to aspire to the trappings and functions of statehood while lacking real nationality."
"Its people speak 30 languages and dialects and vote for over 100 major parties. Trying to build new institutions or transferring wide-ranging powers from long-standing institutions to new ones will end in disaster if those new institutions do not have the wholehearted consent of those they are supposed to serve."
Mr Howard also told his American audience that the most enthusiastic federalists also wanted to undermine the pre-eminence of the United States in the western alliance.
He said that if there had been a common European foreign and security policy during the Gulf war, there would have been a majority against intervention.
"As an independent country," Mr Howard said, "the United Kingdom was able to send ground and air forces to the Gulf; as part of a European federation, we would have had to abide by the agreed policy.
"The same is true of our support for other US military actions from the Korean War to the bombing of Tripoli and Benghazi in 1985."
Having posed the threat, however, Mr Howard said that he did not want to give the impression that anti-Americanism was "shared by all or even most" Europeans.
"It remains the case that most Europeans still see Nato as the basis of their defence and cherish their links with the United States. If we are to preserve the Atlantic partnership, with all its political, military and cultural benefits, we cannot be complacent."Reuse content