The charity is in discussions with two public-sector general bodies, Unison and the GMB, about representing its staff.
The move comes in response to staff at the Salvation Army raising the issue of union membership with the management.
However, all organisations with more than 150 staff will have to set up some sort of formal employee consultation structure by 2005. As a result charity management boards are increasingly inviting unions to bid to them for formal recognition.
But the charity has been criticised for restricting the discussions to these two unions and for not consulting its staff. Chris Ball, national voluntary-sector secretary at Amicus, which is positioning itself as the main union for the sector, said: "It's not for management to decide which union the workers should join - if you give them an unfettered choice, they'll make a good decision."
Amicus has now written a letter to the charity requesting a meeting.
The Salvation Army's employees include care workers, social workers, surveyors, drivers and medical staff, as well as marketing, fundraising, policy, finance and campaign professionals.
Len McCluskey, national secretary of the Transport and General Workers' Union (T&G), which has recently finalised recognition deals with Greenpeace and Crisis, said: "The T&G is the largest social economy and voluntary-sector union in Britain, representing 20,000 members in the voluntary sector. We would welcome the opportunity to be invited into any voluntary-sector organisation to discuss representation."
The Salvation Army denies that talks have been concluded and that a choice has been made. "Discussions are ongoing, with no final decision. The Salvation Army will continue to look at its options,
said a spokeswoman.
Talks with the Salvation Army had consisted of one "very preliminary discussion", said Owen Davies, Unison's national officer for the community and voluntary sector. "If these discussions were to come to something substantial that would have to be on the basis of the Salvation Army consulting staff on whether they want a union to represent it and what union that should be. That's Unison's policy,
A GMB spokeswoman said: "The voluntary sector has not traditionally been unionised but it is a sector whose members would benefit from joining a union."
Traditionally, there has been low union recognition in charities and at present only around 15 per cent of the UK's 500,000 voluntary-sector workers are union members.