Robert Biedron said his party would stand alone in Poland's national elections later this year

WARSAW, May 28 (Reuters) - Poland's third-largest political group, the new leftist Wiosna (Spring) party, said on Tuesday it would stand alone in national elections, dismissing calls to join an opposition alliance to take on the ruling nationalists.

Its leader Robert Biedron - Poland's first openly gay lawmaker - said members had made the decision on Monday, the day after winning their first three seats in the European Parliament with 6% of the vote.

"We've decided to make an independent start," Biedron said, as he responded to journalists asking whether he would accept opposition invitations to join the European Coalition (KE) alliance in elections scheduled for October and November.

He did not spell out his reasons. But his party has accused both the opposition coalition and the ruling Law and Order (PiS) grouping of being too close to the powerful Catholic church.

PiS surged to a clear victory in the European Parliament election with 45.6% of the vote - its best ever performance and a stamp of approval for the its nationalist, eurosceptic platform.

The European Coalition, comprising the Civic Platform formerly led by European Council President Donald Tusk and a group of leftist and rural politicians, came second with 38.3%.

At current levels of support, PiS would have still enjoyed an overall lead if Wiosna had joined the coalition.

Biedron, the 43-year-old former mayor of the northern city of Slupsk, launched his party in February, promising to liberalise strict abortion laws, allow same-sex marriages, tax the Catholic Church and phase out coal by 2035. (Reporting by Marcin Goclowski; Editing by Andrew Heavens)

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