The Constructors Liaison Group is publishing a handbook that claims retentions are illegal under the terms of the Construction Act.
Rudi Klein, chief executive of the CLG, said it will suggest that firms with outstanding retentions should take cases to adjudication.

He said that he knew of five firms of solicitors prepared to take up such actions for a fixed fee.

The handbook is part of the CLG's campaign against retentions – money owed to subcontractors and contractors but withheld to cover defects found after a job is finished.

He said: "The handbook will apply bottom-up pressure on the main contractors."

Andrew Smith, the chief secretary to the Treasury, told a CLG conference last week that the government was aware of the campaign to abolish retentions.

He said the CLG was responding to the government's challenge to the industry to identify and eliminate the most common causes of defects. He said: "By tackling the level of defects on a project, the underlying cause or demand for retentions will be removed."

The CLG has taken its campaign to the House of Commons where Labour MP for Crosby, Claire Curtis-Thomas, is expected to table an early day motion in the new year.

The CLG aims to obtain the signature of 200 MPs to the motion. It has also laid a motion for debate in the Scottish parliament.