SCOTTISH ministers are gearing up for a constitutional war with the UK Government over controversial plans to curtail trade union rights.

The London and Edinburgh administrations are at loggerheads over whether Holyrood should have the right to give consent to provisions set out in the Trade Union Bill, which is being pushed through by the Tories in Westminster.

SNP ministers believe that aspects of the proposed law, which has been condemned by opponents as an ideological attack on workers' rights, directly impact on devolved responsibilities, meaning the Scottish Parliament should have the right to block it north of the Border.

However, UK ministers say that it focuses on employment rights and industrial relations - reserved to Westminster under the devolution settlement - and have insisted they will press ahead regardless of opposition in Scotland.

It leaves the two administrations in an unprecedented constitutional stand-off, with the Scottish Government hoping that Scottish Parliament lawyers will back up its position and that presiding officer Tricia Marwick will support a legislative consent memorandum that it laid at Holyrood.

The feud opens up the prospect of a constitutional crisis, which could see the Scottish Government refuse to enforce a law in an area that the UK Government insists it has sole control of if it is forced through.

Senior figures at the UK Government hinted that they believed the SNP was playing politics with the issue by using an unpopular piece of legislation to demonstrate the potential benefits of independence.

It comes as both First Minister Nicola Sturgeon and UK Labour leader Jeremy Corbyn prepare to speak at a Scottish Trades Union Congress rally against the legislation in Glasgow tomorrow.

The Trade Union Bill, currently making its way through the House of Lords and yet to receive final approval in the Commons, would introduce minimum turnouts in strike ballots, allow employers to use agency workers to replace striking staff and end the so-called check-off system for collecting union subs direct from a salary. Strike organisers would have to wear armbands to identify themselves on pickets.

Roseanna Cunningham, the SNP's Cabinet Secretary for Fair Work, Skills and Training, said the Scottish Government was clear in its view that the Bill will have an impact on devolved functions and that legislative consent should be required.

She added: "There is a clear case for Scotland to be removed from the Bill or, at the very least, for the regulation-making powers to be confirmed as within the devolved competence of Scottish Ministers.

"While the UK Government maintain that the bill is reserved, we have asked the Parliamentary authorities to consider a Legislative Consent Memorandum asking the Scottish Parliament to withhold its consent for the Bill.

"I hope that the requirement for legislative consent will be accepted by the Parliament, but if that is not the case we will ensure that the Parliament has the opportunity to consider a general memorandum and express its opposition to the Bill."

At a Holyrood debate last month, only the Conservatives supported the measures outlined in the Bill. At Scottish Labour's conference last month, a motion calling for the Scottish Government to withhold consent was passed unanimously, with many delegates openly advocating acts of civil disobedience to oppose it. A series of councils have also voted to ignore the legislation if it is passed.

A senior UK Government source said he believed the Scottish Government's position was "more about politics than the law".

He added: "On the legal side, it's pretty straightforward. If the UK Parliament passes a law on Trade Unions it applies in Scotland. But the Scottish public may not be impressed by that law. Then you enter the a discussion over what they want to do about it."

A UK Government spokesman said: "The Trade Union Bill relates to employment rights, duties and industrial relations, all of which are clearly reserved matters for the UK Parliament under the Scottish devolution settlement."