The Tories are urging legal action against minimum alcohol pricing, despite supporting the Scottish Government plan.

The party wants other EU states to challenge the policy to ensure a quick decision on whether it meets free trade rules.

The Alcohol Minimum Pricing Bill will enforce a 50p per unit floor price on units of alcohol, pushing up the cost of cheaper, stronger drinks.

The bill is expected to clear the final parliamentary hurdle in a vote at Holyrood on Thursday.

Conservative health spokesman Jackson Carlaw said: "This is not an attempt to thwart the Bill but rather to ensure that every effort is made to determine whether the measure is legal."

His party agreed to back the policy after securing a voluntary commitment by the Scottish Government to notify the European Commission (EC) of its plan.

The EC might not give a ruling unless there is a challenge from one of the 27 EU member states, said Mr Carlaw.

He said: "It is vital that minimum pricing is given a robust MOT to ensure it does not break EU free trade rules.

"So, to ensure that the EC expresses a view, Scottish Conservatives intend to meet and encourage concerned member states to mount a challenge so we can have clarification from Europe on the legality of a 50p unit price for alcohol.

"This process has shown how the Scottish Conservatives can deliver for Scotland in the face of a majority SNP Government.

"If it works then we will be delighted that we aided that success. If it fails then we have secured the mechanism by which it can be dropped."

A so-called sunset clause was also secured by the Tories, meaning the policy could be ditched in six years if it does not work.

Mr Carlaw will travel to Brussels next month for a series of talks with industry figures and politicians from member states in Europe. Legal action could then be launched within a window of three to six months, he said.

Concerns that the policy is illegal were raised with Westminster's science and technology committee in October last year.

UK public health minister Anne Milton told the committee: "Minimum unit pricing, I think, is an expression used, not by this committee, slightly carelessly sometimes by others.

"Our advice is that, in itself, it is probably illegal as it contravenes European free trade legislation.

"I know Scotland is thinking about introducing it, and they will be challenged, and that will clarify the law. But our advice is that it is illegal."