David Cameron has hammered the "final nail in the coffin" of the Tory modernisation project by agreeing to work with far-right parties in Europe, Labour has said.
Members of the Tories' European Conservatives and Reformists (ECR) group in the European parliament voted to admit the Danish People's party and the True Finns last week.
The move means the grouping, which was founded by Cameron after he broke with the main centre-right European People's Party (EPP), now has 55 MEPs and could soon become the parliament's fourth-largest group.
But it also makes the Tories allies with politicians who have likened the Muslim headscarf to the Nazi swastika and compared multiethnic society to rape, violence and forced marriages.
Some eurosceptics fear the move will drive moderate reformers into the arms of the federalists, as they express horror at the partners the Conservatives have chosen in the European parliament.
In a letter to Tory chairman Grant Shapps, Labour shadow Europe minister Gareth Thomas said the two far-right parties were on the fringes of European opinion and had been shunned by other mainstream centre-right parties, including Angela Merkel’s Christian Democrats (CDU).
"In light of these developments, it is deeply concerning that following the Conservative's decision to admit these two new members, some of the new MEPs in your party's European grouping have criminal records for making inflammatory and socially divisive remarks in their own countries," he wrote.
Thomas was referring to Jussi Halla-aho of the Finns Party, who was convicted of stoking ethnic tensions after suggesting there was a link between Islam and child abuse.
He also referred to Danish People's party MEP Morten Messerschmidt, who earned a criminal record after placing an advert in a Dutch magazine saying: "Mass rapes, gross violence, insecurity, forced marriages, oppression of women, gang crime. This is what a multi-ethnic society offers us."
Thomas continued: "I believe it is vital that the Conservative party is now open with the British public about the dubious, and indeed extremist, views of your new partners in Europe.
"Even in 2009, when David Cameron sought to use his alliances in the European parliament to appease his Eurosceptic backbenchers, he nevertheless rejected an alliance with the Finns party and the Danish People's party because of their extreme views. He must now answer the question of what has changed his mind since then."