By Anna Nicolaou and Luke Baker
BRUSSELS (Reuters) - Prime Minister David Cameron faced new tensions with German Chancellor Angela Merkel on Thursday after his group in the European Parliament voted to accept Germany's anti-euro AfD party into its political family.
The European Conservatives and Reformists, which Cameron formed in 2009 after withdrawing from the European People's Party, Europe's main centre-right group, voted narrowly in favour of letting Alternative fuer Deutschland into the bloc.
The tally of the secret ballot was not released but members said it was 29 votes for, 26 against. Two members of Cameron's Conservatives defied his call to vote against AfD, sources said. Had they obeyed, the German party would have been rejected.
AfD said it had been admitted by a "clear majority" and its leader, Bernd Lucke, was ecstatic about a decision that gives his year-old party of intellectuals respectability and clout.
"Our successful admission is a victory against those who put huge pressure on members of the (ECR) group because they wanted to prevent, for domestic political reasons, the AfD from being recognised and strengthened," he said.
With the addition of AfD's seven members, the ECR will now have 62 seats in the European Parliament, making it the third largest group after the EPP and the Socialists, ahead of the centrist, pro-European liberals.
It may gain a handful more if a small Bulgarian party joins.
But while the ECR has gained in power and influence, Cameron's strength within it has declined and Thursday's decision leaves him in an uncomfortable position with Merkel, whose CDU party is the driving force within the rival EPP.
Cameron, who has promised Britons an in/out referendum on EU membership in 2017, needs Merkel as an ally if he is to get a candidate acceptable to Britain as president of the European Commission and if his drive for a market-friendly, decentralising reform of the EU is to move ahead.
Although AfD is not opposed to the European Union, it has suggested weaker members should leave the single currency and it disagrees with Merkel's broader European policy. It also opposes a planned free-trade agreement between the EU and United States.
SECRET BALLOT
Cameron's spokesman said the prime minister was "very disappointed" by the decision and had made clear to Merkel and others that he did not support AfD's membership.
A Conservative spokesman said Cameron would make every effort to work closely with Merkel's CDU-CSU, which he referred to as the Conservatives' "sister party".
The vice-chairman of the CDU, Armin Laschet, said the AfD, which is becoming a serious challenge to Merkel on the right, had exposed its true nature by joining the ECR group.
"It is working with anti-German Polish nationalists in a group that is in favour of Turkish membership of the EU," he told Reuters. "This is AfD's first breach of its word to its voters."
Cameron faces a tough job explaining to Merkel what line the ECR will take on policy issues such as free trade and market liberalisation, and whether he can control a group that is at sharp odds with the main centre-right bloc.
As well as the AfD, the group has taken in the far-right, anti-immigrant Danish People's Party and the Finns, a nationalist Finnish party that has shaken up domestic politics.
Thursday's decision may also make it harder for Cameron to convince Merkel that Jean-Claude Juncker, the EPP's candidate to be European Commission president, is the wrong man for the job.
Merkel has stood behind Juncker while trying to accommodate Britain, but Cameron is trying openly to rally enough support to block the former Luxembourg prime minister, a veteran deal broker whom he sees as an unacceptable European federalist.
The vote on admitting the AfD was meant to take place last week but was postponed, a sign that Cameron was uncomfortable with rocking the boat while he negotiated over Juncker.
The fact that it went ahead on Thursday and was not delayed again suggests that negotiation effort may have failed.
Admission to the ECR confers international respectability on a party created little more than a year ago by a group of conservative professors opposed to bailing out troubled members of the euro zone. The AfD will get a share of the extra funds, staff and speaking time granted to groups in the EU legislature.
In Berlin, a senior figure in Merkel's Christian Democrats played down the significance.
"That a rival party joins another grouping in parliament is completely normal. This will have no negative consequences," said Gunther Krichbaum, chairman of the German parliament's European affairs committee.
However he added a warning to Cameron: "David Cameron would be well advised to put more emphasis on the advantages of EU membership for Great Britain."
Another senior CDU official, Thomas Strobl, said it would certainly not make cooperation between the EPP and the ECR group in the European Parliament easier.
(Additional reporting by Noah Barkin and Stephen Brown in Berlin and Andrew Osborn in London; writing by Luke Baker; editing Paul Taylor)