It has been revealed companies will have to pledge to "build good relations" with unions when they bid for a slice of £115 billion annual spending on public services.
The rule is part of a deal between ministers, business leaders and workers' organisations due to be unveiled by the Cabinet Office today.
The details will be seized on as evidence of the trade unions' growing influence over Labour, with the party dependant on their donations to stay afloat financially.
The new requirement is expected to apply to public-service delivery contracts awarded by central government rather than to suppliers of equipment or other goods.
Cabinet Office minister Ed Miliband has been leading negotiations over the joint statement, which have been going on for more than a year.
A spokeswoman for the department said the deal merely guaranteed workers "basic rights" - such as being informed that they could receive support from unions. It would not oblige firms to encourage staff to join unions, she insisted.
Prime Minister Gordon Brown's spokesman said today's document did not change the law or introduce any new rights for workers.
He told reporters at a regular daily briefing: "What is being published this morning is a joint statement agreed by Government, trade unions, the CBI and third sector organisations.
"We are asking employers to ensure that employees are aware of all their employment rights."