Big public sector unions are urging councils and other public sector bodies to negotiate fair wages clauses into their arrangements with contractors.
The unions will gear up for a campaign to coincide with the Labour Party and TUC conferences in the autumn.
The move follows a legal opinion voiced by James Goudie QC, the country's foremost legal expert in procurement.
Malcolm Wing, head of local government at Unison, said: 'It would be legal to add a fair employment clause into contracting procedures. We are very pleased at this result but we want to persuade the government to extend this to make this clause a requirement on all councils.'