The Progressive Party is effectively standing aside at the next election and its members have been told they are free to join Labour if they want to.
Party leader Jim Anderton says in his latest newsletter that working with Labour is the best way promote Progressive Party ideas in mainstream politics.
He said he had discussed dual membership with Labour's New Zealand Council and there were no problems with it.
"This means Progressive Party members are free to join their local Labour Party branch and retain their Progressive Party membership," he said.
A spokesman said the Progressive Party would not run a candidate list at the 2011 election and would not campaign for the party vote.
Mr Anderton is its only MP. He has held the Wigram electorate - previously known as Sydenham - since 1984.
In last year's election the Progressive Party won less than one percent of the party vote.
Without Anderton's grip on Wigram it would need to break the five percent threshold to hold any seats.
The decision not to run a candidate list in 2011 means that Anderton, who has turned 70, will continue to be its sole MP - if he decides to stay in politics and if he again won Wigram.
He has not announced his intentions but since the election there has been speculation that this could be his last term in Parliament.
Anderton said in his newsletter dual membership would allow Progressive Party members to seek positions within the Labour Party.
"Some members of the Progressive Party who have been working with Labour on campaigns have been invited to hold office in branches," he said.
Anderton first won the electorate seat as a Labour MP.
He resigned from Labour in 1989 and formed New Labour, which later became part of the Alliance Party.
He was leader of the Alliance until 2002 when he left it to form the Progressive Party.
The party was in coalition with Labour and Anderton was a senior minister before last year's election.
He took the unusual step of maintaining the coalition in opposition and is opposition spokesman on agriculture.
