The Transport and General Workers Union (T&G) says it will launch industrial action against any voluntary organisation providing contracted out services for a local council that does not match a recent pay deal negotiated for directly employed council staff.
The deal secured an 11 per cent rise over two years for lowest paid council staff and a £5 an hour minimum wage. It will increase the overall pay bill by 7.83 per cent.
T&G members have until 20 September to vote on the proposed deal. If it is accepted, the union says it will turn its attention to private-sector contractors, housing associations and voluntary-sector employers.
The T&G is strongly represented in voluntary sector care homes, housing associations such as English Churches Housing and large charities such as Age Concern. The union also has around 20,000 members in the voluntary sector.
"This autumn will see the T&G lead in ending once and for all the shameful treatment of those who provide public services but are not directly employed.
If local government has suffered from low pay then its plight is even worse,
said Jack Dromey, national organiser at the T&G.
He added that the union would first ask voluntary-sector employers which follow local government (NJC) pay scales to honour the recent agreement.
It will then turn to "areas of worst abuse" - employers which pay significantly less than the national local government agreements.
"Once the pay deal has been agreed we will ask all the employers of tens of thousands of our members in the private and voluntary sector to pay up," said Dromey. "We believe that the great bulk will do so readily. However, those employers that fail to honour national agreements will be subject to industrial action."
Valerie Barrow, chief executive of the Association of Charity Officers, the umbrella body for voluntary care homes, said that underpayment of fees by local councils meant that "any increases will be a problem. But we don't have the lowest paid workforce. Members try to keep a stable workforce."
But other voluntary sector trade unions do not seem keen to follow the T&G's militant lead. Owen Davies, voluntary-sector national officer at Unison, said: "We will be pressing for a percentage increase for the voluntary sector that is comparable to the going rate in public sector, but each set of negotiations will addressed on its own merits. It's too early to say if there will be industrial action."
Chris Ball, voluntary-sector secretary at Amicus, said: "With funding the way it is for small charities, if they are forced to give this pay rise, they could lose posts."
Amicus is recommending that charities that follow NCJ rates should offer 3-3.5 per cent together compensatory improvements to paternity pay and extra child care.