“This is a new phase in Swedish politics,” said Sahlin at a mutual press conference.
The immediate goal of the two parties’ enhanced cooperation is to create a coalition government in 2010, but it is also meant to have a long-term view running through 2020.
The Left Party had not been informed in advance of the joint Social Democrat-Green initiative.
The leaders of the three centre-left parties last met in June to discuss economic policy.
At the meeting, the Greens and the Social Democrats put pressure on the Left Party to accept a budget ceiling and the goal of achieving a budgetary surplus.
Left Party leader Lars Ohly refused to accept the ultimatum, but wanted to continue discussions.
This autumn, Ohly has met with Sahlin, but not in the presence of Green Party spokespeople Peter Eriksson and Maria Wetterstrand.
The Greens and the Social Democrats plan to appoint five working groups this autumn which are to generate ideas for long-term policies through 2020.
Their work will also create the base from which a governing platform can be drawn in time for the 2010 parliamentary elections.
“But we don’t have any plans to combine our parties, naturally,” said Wetterstrand.
Sahlin and the two Green Party spokespeople also planning a joint tour around Sweden later in the autumn which will include stops in Gothenburg, Norrköping, and Malmö, among other destinations.