The leader of Sweden's main opposition party on Friday announced she was stepping down after she suggested her alliance accept support from an anti-immigration far-right group.

Criticised for months by her own party, the Moderates, whose public support has plunged in polls, Anna Kinberg Batra's announcement comes a year before a general election and raises questions about who will succeed her. 

"I can take criticism... it's my job... but I don't think the voters who are doubting the Moderates are especially impressed by us in this situation," she told reporters in Stockholm. 

The 47-year-old Batra's mission was to lead a centre-right alliance between the Moderates, the Centre party, the Christian Democrats and the Liberals to victory in the September 2018 vote. 

But the anti-immigration Sweden Democrats, which has hitherto been shunned by the mainstream parties because of its roots in the neo-Nazi movement, came second in a June opinion poll -- ahead of the Moderates. 

The Sweden Democrats have enjoyed a swift rise in public support in recent years, capitalising on Swedes' frustration over immigration, segregation, crime and security.

They first entered parliament in 2010 with 5.7 percent of votes, rising to nearly 13 percent in the 2014 election.

Batra broke a longstanding taboo in January when she opened the door for a cooperation with the Sweden Democrats to topple the minority government led by the Social Democrat Prime Minister Stefan Lofven. 

"It was the right thing to do. I regret nothing," Batra told reporters when asked if she regretted the move, which sparked a deep rift within the opposition alliance. 

"Tackling troublesome issues also means taking a few punches."