JEREMY Corbyn should “get off his high horse” and join fellow opposition leaders in their soft Brexit alliance, Ian Blackford has insisted.

But the SNP’s Westminster leader also acknowledged that without his Labour counterpart taking part in the grouping, it is likely to fail.

Mr Blackford’s appeal to Mr Corbyn comes as he and his fellow leaders from the Liberal Democrats, Greens and Plaid Cymru prepare to table an amendment to the EU Withdrawal Bill at its Report Stage next week, calling for Britain to stay in the single market and customs union.

It will be seen as an attempt to flush out Labour rebels. Last June some 50 Labour MPs defied Mr Corbyn on an amendment to the Queen’s Speech tabled by Chuka Umunna, the former Shadow Business Secretary, which also called for the UK to stay in the single market.

Earlier this week, the single market summit in the Commons empty-chaired Mr Corbyn, who dismissed the initiative as a “stunt”.

Mr Blackford told a press briefing at Westminster that he “despaired” at the Labour leader’s position, noting: “I am utterly convinced we can get somewhere on this but we will only get somewhere if Labour, from Jeremy, decide they want to come to the party. We need to be aware of the risks we collectively face; of crashing out of the EU without having an effective trade deal…

“There is a real opportunity to protect jobs, investment, living standards and workers’ rights but we are going to have to work together.”

Mr Blackford forcefully dismissed the notion that the soft Brexit alliance was a stunt, noting how it would meet monthly and had invited Sadiq Khan, the Labour Mayor, Carwyn Jones, the Welsh First Minister, and Frances O’Grady, the TUC General Secretary, to attend. It is also expected at some stage that Nicola Sturgeon, the First Minister, will also be invited to attend one of the meetings.

“We want to create a unified front to win the argument in the Commons but also win the argument through the nation states of the UK. We need Labour to understand the seriousness of the situation and recognise all of us have to work together. That offer to the Labour frontbench and to Jeremy remains on the table.”

The Highland MP claimed there were many Labour politicians who were "horrified" at Mr Corbyn's lack of engagement and added: "If we are going to ask Conservatives to support us...they will only respond to a united approach across the opposition and Jeremy has really got to get off his high horse. This is not a stunt, it's desperately serious and is about protecting the interests of all our constituents."

Meanwhile during Prime Minister’s Questions the Nationalist leader took Theresa May to task over the failure of the UK Government to table, as promised, amendments to the EU Withdrawal Bill to allay fears of a power-grab by Whitehall when powers are repatriated from Brussels post Brexit.

On Tuesday, David Mundell, the Scottish Secretary, said it was regrettable the amendments could not be tabled in time for next week’s Report Stage but stressed how changes would be made.

The PM told Mr Blackford that the UK Government was working with the Scottish Government to bring forward the promised amendments in the House of Lords.

But Mr Blackford replied: "That simply is not good enough. The Secretary of State for Scotland promised a powers bonanza for Scotland and that, crucially, amendments would be tabled ahead of next week's debate. Yesterday it was revealed that no amendments would be launched.

"The Tories always promise Scotland everything and deliver nothing,” he declared.

Earlier, David Lidington, Mrs May’s new Whitehall fixer, said he was committed to "intensifying" discussions with Edinburgh on avoiding a constitutional crisis over Brexit.

The Cabinet Office Minister, who has already spoken to John Swinney, the Deputy First Minister, has offered to chair a Joint Ministerial Committee meeting in London and to hold fresh bilateral talks in Edinburgh.

It is also expected in February the PM will chair a plenary session of the JMC attended by Ms Sturgeon.