The ACT branch of the Community and Public Sector Union moved to affiliate 9000 members this week with the ACT Labor Party a move that could set off a firestorm of protest.

And the move to affiliate the public servants' union with the local branch of the ALP could have a profound effect on the preselection of candidates before the next election.

Although more than 12 unions are already affiliated with local Labor including the powerful Construction, Forestry, Mining and Energy Union and the Transport Workers Union the move by the public sector union could significantly change the balance of power in the party, especially on its Left.

A senior ALP source confirmed the move would have a ''huge'' impact on the ''internal dynamics'' of the party.

And he warned it could affect the standing of the union in the eyes of local members.

''Even people in the public service who are Labor-leaning, even if large numbers believe in Labor, they don't think there should be affiliation. The professional standing of the public service is affected by this the affiliation would compromise members,'' he said.

''This has always been considered a 'no-go' because the public service has to serve Liberal governments, too.

''But as long as they [the CPSU] are a legitimate union, I don't think Labor can reject the affiliation.''

Acting CPSU national secretary Mark Gepp said the union had decided in early 2007 to ''affiliate progressively'' with each state-based branch of the ALP. He said the decision was taken to give the union the capacity to ''win improvements for our members through the political and parliamentary process''.

And he stressed that the union gave members the chance to opt out of affiliation with Labor.

ACT Opposition Leader Zed Seselja said the move would concern many Canberra public servants.

''First and foremost, there is a valuable tradition of the public service being impartial and separate from the party political process. Clearly, that could be called into question if the public servants' union were officially affiliated with one of the parties,'' he said.