French National Front leader Marine Le Pen has formed a political group of right-wing nationalist parties in the European Parliament.
The anti-EU Europe of Nations and Freedom bloc includes Italy's Northern League and the Freedom Party of Dutch politician Geert Wilders.
The grouping also has the support of UK MEP Janice Atkinson, expelled from UKIP in March over an expenses scandal.
Forming the group will give the MEPs more influence in the parliament.
It will also mean that the new bloc's members have access to millions of euros in extra funding as well as more staff and speaking time.
To be valid, a group needs 25 MEPs from at least seven different nationalities.
Mr Wilders, whose anti-Islam rhetoric has seen him face legal action, said that the formation was a "historic moment".
"Today it's the beginning of our liberation, our D-Day," he told reporters, adding that the new bloc would be the voice of the new "European resistance", defending their countries' sovereignty. The anti-Islam leader said the bloc would fight mass immigration as well as "Islamisation".
Eurosceptic and far-right parties made gains in last year's European Parliament elections, in what France's PM Manuel Valls called a "political earthquake".
Until now the far-right National Front (FN) has struggled to find members from enough countries to form a group.
But it secured support from MEPs in Italy's Northern League, Austria's Freedom Party, Vlaams Belang from Belgium and the Polish Congress of the New Right.
The new group represented "a political force that will go far beyond our previous situation," Ms Le Pen told reporters in Brussels on Tuesday.
She said it would be "strong, determined, coherent and ambitious".
The group could qualify for around €17.5m (£12.6m) of EU money over the next four years, according to calculations by the think tank Open Europe.
The FN leader was praised by Ms Atkinson, who said Ms Le Pen was "leading the offence" against what was happening in the European Union.
Ms Atkinson was expelled by UKIP for "bringing the party into disrepute" amid newspaper allegations that a member of her staff tried to arrange a false receipt for a meal.
UKIP has previously said it was "not interested in any deal" with Ms Le Pen or her party because of "prejudice and anti-Semitism in particular" in the FN.
UKIP leader Nigel Farage already heads another anti-immigration alliance in the European Parliament called the Europe of Freedom and Direct Democracy Group (EFDD).
The party responded to Ms Atkinson's move to join the rival group on Tuesday by saying: "Whatever she does is beyond our control."
FN founder Jean-Marie Le Pen, who was suspended from the party last month over his comments that the Holocaust was "a detail of history", is not part of the new Europe of Nations and Freedom group. FN MEP Bruno Gollnisch is also not a member, according to reports.
Marine Le Pen, who succeeded her father as FN leader, has spent the past year trying to distance the party from his remarks, widely condemned as anti-Semitic.
Meanwhile the three MEPs from Greece's far-right Golden Dawn party remain unattached to a political group in the European Parliament.
Correction 19 June 2015: This story has been amended to delete mention of Hungary's Jobbik party, which is not in the new group.