UKIP has teamed up with other Eurosceptics to form a new 30-strong group in the European Parliament.
The group, called Europe of Freedom and Democracy, is headed by UKIP Leader Nigel Farage.
Incorporating nine MEPs from Italy's anti-immigration Northern League, the new alliance replaces the Independence and Democracy group.
UKIP's 13 MEPs form the biggest component of Europe of Freedom and Democracy but ther other members of the new group come from the old Independence and Democracy bloc, apart from a newly-elected Finn.
The Northern League formerly sat in the conservative UEN bloc, which no longer exists.
With 30 MEPs from eight countries the group surpasses the new parliamentary minimum of 25 MEPs from seven countries needed to qualify as a formal group.
UKIP spokesman Gawain Towler said UKIP was the only group member calling for withdrawal from the EU, "but they are all sympathetic to UKIP's position".
He said that Europe of Freedom and Democracy would provide "real opposition" to European federalism.
"The British Conservatives' new group, European Conservatives and Reformists, which espouses an anti-federalist agenda, is in fact part of one giant coalition, all going in the same direction," Mr Towler said.
The new UKIP-led group's members are from Italy (nine), Denmark (two), Greece (two), Finland (one), France (one), the Netherlands (one) and Slovakia (one).
UKIP won 17.4% of the British vote and increased its number of MEPs to 13 in the June election - beating Labour into third place. UKIP had 12 seats in the former parliament.
There will be a comprehensive page devoted to the new group as soon as possible.