British Conservative Martin Callanan has been elected president of the European Conservatives and Reformists group (ECR) in the European Parliament - the group formed after David Cameron pulled his MEPs out of the mainstream centre-right bloc in Strasbourg.
Mr Callanan, the first Briton to take charge, was voted in to replace the Czech Republic's Jan Zahradil, who is stepping down.
Mr Callanan has been leader of the UK Conservative faction within the group for a year and takes over a day after the Prime Minister was lambasted in a two-hour Strasbourg debate for blocking an EU Treaty change.
He said he intended to continue the work of the ECR group in carving out "a clear vision for a more open, flexible and competitive European Union".
He said: "The ECR's message has never been more important. Our agenda of Euro-realism is striking a chord with the European people who are becoming fed-up with their leaders putting their economies at risk in order to pursue their political projects.
"Under my chairmanship, the ECR will continue to promote initiatives which champion competitiveness and the single market, and which encourage strong cooperation between national governments. But we will strongly oppose efforts to create a federal super-state of Europe."
Mr Cameron's decision to move the Tories out of the European People's Party (EPP) - on the grounds that it was too federalist - risked isolating Conservative MEPs.
It meant Mr Cameron's own exclusion from meetings of EPP national party leaders - including a vital pre-summit meeting last week which some say would have given him better advance warning of the impending summit rebuff and given time to change tactics to secure UK concessions.
And it meant Tory MEPs had to find new political bedfellows in a parliament where cross-national allegiances are vital.
Mr Callanan rejected claims that the Tory move to the newly-created ECR in 2009 had been an early sign of EU isolation under Mr Cameron.