UKIP's Nigel Farage will lead the new group initially
The UK Independence Party (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, it is further to the right than Independence and Democracy - the group it replaces.
The old group - unlike the new one - had a few centre-left Eurosceptics.
UKIP's 13 MEPs form the biggest component of Europe of Freedom and Democracy.
The 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 told BBC News that Europe of Freedom and Democracy would provide "real opposition" to European federalism.
He said the British Conservatives' new group, European Conservatives and Reformists, which espouses an anti-federalist agenda, was in fact part of "one giant coalition, all going in the same direction".
The new UKIP-led group's other members are from 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.