The Eurosceptic Dutch politician Geert Wilders and French National Front (FN) leader Marine Le Pen have launched what they call an "historic" alliance for next year's European elections.
Mr Wilders said they had agreed on the need to repatriate from Brussels the power to control their countries' borders and economies.
They held strategy talks in The Hague.
Both leaders say Europe's political elite has been too tolerant of Islam and both want to curb immigration.
Ms Le Pen, a Euro MP, visited the Dutch parliament with Mr Wilders. His party lost almost half of its seats in the September 2012 Dutch election, but it is doing well now in opinion polls.
Eurosceptics are widely expected to make significant gains in the European elections in May, as debt-laden EU countries struggle to revive their anaemic economies.
"The time of patriotic movements being divided is over," said Ms Le Pen, calling it "an historic day".
"Today is the start of the liberation of Europe from the monster of Brussels," said Mr Wilders, who heads the Freedom Party (PVV).
Historically, nationalists and other anti-immigration parties have been fragmented in Europe. They tend to campaign on national issues, rejecting EU integration and any further weakening of national sovereignty.
Mr Wilders acknowledged that the UK Independence Party (UKIP), led by Nigel Farage, was not yet willing to join the new alliance.
"I understand that he [Mr Farage] is not too eager today to work with my party, but let me tell you, I hope after next year's elections he will be able to join in our initiative," Mr Wilders said.
The PVV and FN currently sit among independent MEPs in the European Parliament. They are not in UKIP's group.
To form a new officially recognised bloc they would need a minimum of 25 MEPs from at least seven EU member states.
Correspondents say the two parties could team up with groups such as Austria's Freedom Party (FPOe), Italy's Northern League, Vlaams Belang in Belgium, the Sweden Democrats or the Danish People's Party.
Recognition as an official political group in the European Parliament gives group members EU subsidies, offices, assistants and seats on parliamentary committees.