David Cameron has been criticised by a senior US Republican senator over his decision to pull the Conservative party out of the main centre-right political group in Europe.

John McCain, a possible contender for the US presidency in 2008, said he viewed the European People's party - which includes the main Christian Democratic parties in the European Union - as a vital partner for the Republicans.

Speaking in Brussels, Mr McCain praised Mr Cameron for making the Conservatives more "competitive" but suggested the Tory leader should show some solidarity with the party's European allies.

"I do give them great credit for reinvigorating the Conservative party," he told the Financial Times at a lunch given by jointly by the EPP and the International Republican Institute, which he chairs.

But he added: "I would hope they would appreciate the support they received from the EPP when they were wandering in the wilderness."

Behind Mr McCain's comments is a fear in Republican circles that Mr Cameron's strategy could isolate the Tories from the centre-right political mainstream in Europe. He said the Republicans viewed the Tories as important interlocutors.

In particular, the Bush administration is concerned over the level of regulation being created by the EU which has an impact on US business, notably recent legislation on the testing of substances produced by the chemical industry.

The Tories are viewed in Washington as a bulwark in the European parliament against red tape, and their members have played a key role in defending liberal principles.

Although Tory MEPs would still fight against regulation if they sit in a new political group, they would lose some of the important committee posts which give them greatest ability to influence legislation.

For example, Malcolm Harbour, a Tory, played an important role in maintaining some liberal momentum in new legislation affecting the EU services sector.

Mr Cameron wants the Tories to leave the EPP group, which he regards as too federalist. The group supports the euro and the European constitution, although the Tories negotiated a deal allowing them to maintain their own positions on constitutional issues.

His search for potential allies for the new group has taken him to the fringes of European politics. Talks have been held with a Christian Dutch party which does not allow women to stand for parliament and Italy's Alleanza Nationale, a centre-right party with neo-fascist origins.

However, his main allies are likely to be Poland's populist Law and Justice party and the eurosceptic ODS party in the Czech Republic.

James Elles, a Tory MEP, said he expected Mr Cameron to press ahead with plans to create the new group, but warned it would split the party in Brussels. He added: "Of course the Republicans will continue to talk to Conservatives but Senator McCain's comments are relevant. The more you are part of a broader network, the more influence you have in shaping the agenda."

The Conservatives have always had a difficult relationship with the European People's party, whose national components tend to favour EU integration and are more supportive of the European social model.

Michael Howard, the former Tory leader, toyed with the idea of leaving the EPP but ended up striking a deal with Hans-Gert Pöttering, the German group leader, which left the Conservatives with a great degree of independence.

Mr Pöttering has been highly critical of Mr Cameron's decision to pull out.