"Commissioners may be active members of political parties as long as this does not compromise their work as commissioners," said commission spokeswoman Pia Ahrenkilde-Hansen at Monday's press briefing in Brussels.
The new leader of the Social Democrats, Mona Sahlin, announced in her debut speech that Wallström and former foreign minister Jan Eliasson would form a working group to decide the party's foreign and European policies ahead of the elections to the European Parliament in 2009 and the general election in 2010.
Wallström's new role has come in for criticism from government politicians. Liberal Party member of parliament Carl B Hamilton said that Wallström's two roles are incompatible.
"It is unacceptable and incredible to believe that as long as Margot Wallström is a European Commissioner she can also work for the opposition in Sweden and against the sitting government, for example ahead of the European Parliament elections in 2009," said Hamilton in a statement. He demanded that Wallström choose between being a party worker and a commissioner.
Prime Minister Fredrik Reinfeldt also questioned whether it was possible for Wallström to combine the roles.
Wallström's own spokesman, Mikolaj Dowgielewicz, said she had so far not informed Jose Manuel Barroso, president of the European Commission, adding that she planned to do so very soon.
Dowgielewicz added that there was no conflict of interest between the two roles.
"It is an informal working group and Commissioner Wallström will not put herself in any situations that could compromise her duties as commissioner," he said.
The Barroso Commission's mandate expires in 2009, after which governments in EU member states will appoint new commissioners.
It is thought unlikely that Wallström would be re-appointed by the current government. Social Democratic governments have always appointed members of their own party. While the centre-right parties criticized the practice, it is thought that the government will decide that it is the turn of a centre-right candidate to fill the post.