In a joint statement delivered this morning, ministers, unions and business leaders will announce that companies must demonstrate how they will work with unions and provide access to training for basic skills staff as part of their contract bids.

TUC general secretary Brendan Barber said: "Soon any people employed by contractors who win government contracts will be able to access basic skills training at work, be able to find out how to join a union and learn more about the law and how it relates to their job."

However, the move has been met with caution by UK printers. Alastair Smith, group sales director at NEMC, said that the move was "just another hoop to come through".

He said: "Applying for a public tender is a hugely costly and time consuming process. On the one hand the government is saying that it wants to open up contracts to SMEs, on the other it is creating more conditions and a higher barrier of entry.

"We are a non-unionised company but have great working practices in a safe environment with above average pay so why should we have to jump through another hoop?"

The move has been met with criticism from the Conservatives who have said that it is further evidence of "policy favours" from the government in return for "bank-rolling a nearly-bankrupt Labour Party".

Unite recently agreed to underwrite Labour's accounts as financial support for the party dwindles. In an interview in today's Independent, Dave Prentis, secretary general at Unison said that the union was considering its £1.5m a year support of the government.