The centre-right EPP is the largest group in the parliament

Mr Cameron has prompted opposition from some MEPs by saying it is wrong for the Tories to ally with the EPP because of its federalist views.

Mrs Merkel has written to Mr Cameron saying the EPP should be the base of relations between their two parties.

But a spokesman for the Tory leader said his position remained unchanged.

The appeal came in a letter congratulating the Tory leader on his new job.

Mrs Merkel says: "I am sure that you will successfully achieve the renewal of your party in terms of personnel, content and structure, and that in the foreseeable future the great tradition of British Conservatives will once more have a good chance to go into government.

"In this sense, I look forward to good and intensive cooperation with you, and in particular within the framework of the EPP-ED (between the EPP and the British Tories) as a clear base for our partnership and bilateral dialogue."

Threat?

Mrs Merkel said Britain and the British Conservatives were an "indisputable element of European politics".

She added: "In the EU we can work together on our common interests and convictions to safeguard individual freedoms, the regulation of the Common Market, as well as a strong trans-Atlantic cooperation."

Her letter is being seen by some as a threat not to break links with the Tories if they leave the EPP grouping.

The President of the Party of European Socialists, Poul Nyrup Rasmussen, said: "It looks like David Cameron's pledge to take the Tories out of the EPP has resulted in a threat from Merkel never to speak to them again.

"The question is, if the Tories leave, who will jump ship with them?

"Will it be a ragbag of free marketers and nationalists or will one of the bigger parties also abandon the EPP?"

'Consistency'

Mr Cameron, and shadow foreign secretary William Hague, say they will try to form an alternative European grouping - a process which could take months.

His spokesman was unmoved by Mrs Merkel's letter changed nothing.

"The letter doesn't change our position," he said.

"It's our belief that we should be consistent and say the same thing in Westminster as we do in Brussels.

"However, that does not prevent us working with the other EPP nations."

Earlier this week, one senior Conservative MEP, Sir Robert Atkins, said Mr Cameron could not take an "arbitrary" decision.

"He has to come and talk to us and discuss his plans," he added.

Sir Robert said the MEPs would point out some facts, and that it would be possible to reach a "sensible solution".

On Tuesday, the 27 Tory MEPs re-elected their chairman, Timothy Kirkhope, by a two-thirds majority on what one EPP source described as a "stay-in-the-group" ticket.

Mr Kirkhope issued a statement afterwards saying only that he looked forward to a "positive and constructive working relationship with [Cameron)]and his shadow foreign secretary".

The losing candidate, Chris Heaton-Harris, who has long campaigned for withdrawal from the EPP, said he expected the break with the group would be a "relatively quick thing" once Mr Cameron appointed a shadow foreign secretary.