Conservative leader David Cameron has come under fire from a senior European politician over his alliance with "exotic" ultra-nationalist parties in the European Parliament.

The Spanish secretary-general of the European People's Party (EPP), Antonio López-Istúriz, has predicted that the European Conservative and Reformists (ERC), the recently-formed Eurosceptic group, could collapse, allowing Tory MEPs to rejoin the mainstream EPP.

Mr Cameron has been dogged by his connection to his controversial new allies from Poland, Latvia, Hungary and the Czech Republic, who have been accused of holding antisemitic and homophobic views. His warm relationship with the chairman of the Czech Republic's Civil Democratic Party, Mirek Topolanek, was criticised last week when Mr Topolanek resigned after making derogatory comments about Jews and homosexuals.

In a briefing in Brussels this week, Mr López-Istúriz, an MEP from the centre-right Spanish People's Party, said: "They have no influence and they have nothing to say within the EU institutions right now. Being left out of the main political institutions in the EU is a mistake. I believe that David Cameron will make the pragmatic choice after the elections to come back. "

He said Czech members of the new grouping were already planning a return to the EPP, which includes parties which are in power in 13 European countries, including France, Germany and Italy. None of the Tories' current allies forms the party of government in their home countries.

Mr López-Istúriz said that he hoped Mr Cameron would win the forthcoming election, but he risked cutting himself off from senior figures such as Angela Merkel and Nicolas Sarkozy if he insisted on remaining within the new alliance.

"They (the ERC) have some disturbing ideas, not only about Europe but also gay rights," said Mr López-Istúriz. The poor access of the present Tory leadership was in sharp contrast to that of Mr Cameron's predecessors, who, he said, "brought a critical voice but it was inside the family, part of an internal debate".

The Labour Party has been particularly critical of the ECR's Polish leader in Europe, Michal Kaminski, whose views on the Holocaust, Jews and homosexuals were put under the microscope last year when he was the guest of honour of Conservative Friends of Israel at the party's annual conference.

Mr Kaminski has consistently said that he does not think the Polish nation should be held collectively responsible for a massacre of Jews that took place at Jedwabne in 1941. However, he always strenuously denied that he is an antisemite.

Last weekend, the Observer revealed that in 2007 an internal Conservative Party report had warned against an alliance with Mr Kaminski's Law and Justice Party because of its far-right associations. The report, which runs to 200 pages, is said to recommend remaining within the EPP.

Experts on Poland are divided on whether the death of Polish President Lech Kaczynski and members of his inner circle in a plane crash in Russia last weekend will lead to a surge in popularity for the party, or contribute to its further decline.