By Stephen Mulvey
BBC News EU reporter
Conservative supporters of a split with the European People's Party say David Cameron should take the plunge despite problems with a key Czech partner.
Mr Cameron faces strong opposition from pro-European Tories
The ODS party said this week it will not be able to join the Tories in a new European Parliament group until 2009.
Without it, the Tories' main partner could be a Polish party accused of homophobia and links to the far right.
But advocates of a split say Mr Cameron must keep a promise he made before his election as Conservative Party leader.
"The promise was just to leave, to leave the alliance with the European People's Party (EPP), it was not a promise to form another grouping," said David Heathcoat-Amory MP.
"A new group would be an advantage, but if we cannot do that we should sit with the allies we do have."
Mr Cameron promised to pull the party's 27 MEPs out of the EPP group, because its federalist agenda clashes with the Tory vision of Europe.
However, pro-European Conservatives say the party will lose influence if it leaves the largest group in the European Parliament and a number of MEPs are in open rebellion.
Some MPs believe even shadow foreign secretary William Hague opposes the move, according to leaked e-mails to Mr Cameron from his private secretary Desmond Swayne, published in the Sunday Times newspaper.
Mr Swayne wrote that the plan to leave the EPP had backfired, leaving pro-European MEPs "furious" and Eurosceptics anxious that the project will be "blown off course".
If the Tories do leave the EPP they would either have to sit with independents in the European Parliament - alongside French far-right leader Jean-Marie Le Pen - or try to form a group with Poland's ruling Law and Justice party as their main partner.
Neil O'Brien, director of Eurosceptic think-tank Open Europe described it as a "tricky choice".
Sitting alone would be financially costly, he said, because of the way funds are distributed to political groups in the European Parliament.
"On the other hand, people have questioned whether the British Conservatives, who are pro-free market, would fit with the Poles, who are more socially conservative," he added.
Law and Justice has been accused of homophobia, after trying to prevent gay pride marches.
The party has also formed a coalition in Poland with the League of Polish Families, which Israel regards as anti-Semitic.
But Daniel Hannan MEP said the parties the Conservatives are inviting to join a new group were actually "more moderate" than some of those in the EPP.
Italy's Forza Italia party had posted homophobic advertisements in the country's recent election campaign, he said, while a German Christian Democrat had resorted to xenophobic slogans a few years ago in a fight against green cards for Indians.
He added that Austria's People's Party was in a coalition with the far-right Freedom Party.
"We are proposing to keep better company than we are currently keeping," he said.
Conservative MEPs opposed to a break with the EPP declined to comment, citing a "self-denying ordinance" that would remain in force until a speech from the party leadership on the subject later this month.