DAVID CAMERON has been accused of trying to "delegitimise" trade unions after ordering an inquiry into their tactics in the wake of the bitter dispute at the Grangemouth petrochemicals plant last month.

Drew Smith, a Labour MSP who was pictured with protesting members of Unite and their now-infamous inflatable rat during the dispute, claimed the probe was an attempt to discredit the union and its links with Labour.

The party's constitutional spokesman at Holyrood spoke out after Downing Street announced a wide-ranging review, headed by Bruce Carr QC, to investigate the use of so-called "leverage" tactics and claims of intimidation.

First Minister Alex Salmond also condemned the inquiry, accusing the Tories of using it to "seek electoral advantage".

Mr Smith said: "The lesson which ministers should be taking from Grangemouth should be to improve industrial relations and encourage partnership with unions rather than try to delegitimise them.

"Having witnessed a Unite demonstration during the Grangemouth dispute, I am surprised by some of the descriptions which others have used.

"Any case of genuine intimidation is to be condemned but condemnation of demonstration in general needs to be balanced against the right to peaceful protest."

The inquiry will examine allegations that "mobs" of Unite members intimidated directors of the Ineos plant by demonstrating outside their homes.

In a sign of Coalition tensions over the investigation, LibDem Business Secretary Vince Cable made clear he had only agreed to the inquiry on the basis that it would also examine the practices of employers, such as the "blacklisting" of workers.

Mr Smith, who is chairman of the trades union group of Labour MSPs and a member of Unite, was pictured at a demonstration in Fife during the dispute.

He said he came upon the protest, understood to be outside the home of an Ineos director, while out campaigning in the Dunfermline by-election and spent a few minutes chatting with Unite members.

He added: "My experience of a small group of union activists certainly was not intimidatory.

"The reality is that being faced with an ultimatum to accept changes to terms and conditions or be sacked is itself an example of bullying behaviour."

Unite general secretary Len McCluskey has always maintained that leverage tactics - borrowed from environmental protest movements - are not intimidatory and last night a spokesman for the union dismissed the probe.

A spokesman said: "This review is nothing more than a Tory election stunt which no trade unionist will collaborate with."

Paul Kenny, leader of the GMB union, described the review as a "sop to the Tory backbenches."

Mr Salmond said the Scottish Government had not been consulted, adding: "There are of course legitimate issues that should be addressed in relation to industrial relations and the operation of our key national infrastructure.

"However, to allow such an important matter to be presented as a political manoeuvre is foolish and irresponsible. The way UK ministers have approached this issue suggests that their interest is less about industrial relations and almost entirely about seeking electoral advantage."

Cabinet Office Minister Francis Maude defended the review, saying: "Allegations about trade union industrial intimidation tactics are deeply concerning. That's why we need an independent review to get to the bottom of these activities, as well as to look at the role played by government, employers and employees in industrial disputes."