Leading labour movement activists warned yesterday that Prime Minister David Cameron's latest anti-union stunt is a highly dangerous plot to impose even more repressive laws.

The PM has unleashed a six-month "review" into trade union practices amid scurrilous media hype around the Unite union's campaign to save the Grangemouth petrochemical plant.

Bruce Carr QC will consider whether the law needs tightening to prevent trade union "intimidation," declared Tory Cabinet Office Minister Francis Maude.

Liberal Democrat Business Secretary Vince Cable tried to sugar the pill by stating that the review must also consider irresponsible behaviour by bosses, including blacklisting.

As coalition ministers wrangled over the precise scope of the inquiry, left Labour MP Kelvin Hopkins warned that it was simply a vicious Tory attack on the unions.

The Luton North MP dismissed Mr Cable's attempt to put a gloss on the purpose of the inquiry.

He said: "It is time to roll back the Tory tide and restore trade union rights."

Veteran Labour MP Dennis Skinner said the probe was a dangerous attempt to focus attention on the unions in the run-up to the general election.

"The millionaire crowd at Number 10 will do anything to try to divert people's attention from austerity Britain, where directors rake in an extra 14 per cent while trade unionists suffer a pay freeze and big cuts in real wages," the Bolsover MP said.

The government says it is putting a bosses' representative and "a union representative" on the review panel.

But Mr Skinner retorted: "I don't believe any decent trade union representative should serve on this review."

Communist Party general secretary Rob Griffiths warned that it reeked of preparation for a new round of anti-union laws.

"A better idea would be an inquiry into how Grangemouth boss Jim Ratcliffe can hold the whole country and economy to ransom," he said.

Labour MP Grahame Morris said Cameron's anti-union electoral stunt "won't deal with the real scandals of blacklisting, nor will it delve into corruption or abuses of power that truly harm politics and industrial relations."

Scottish First Minister Alex Salmond complained that he was not consulted over the review, which looked like a "foolish" political manoeuvre.