UNIONISTS must put forward options to maintain Scotland’s relationship with the European Union or accept that a Brexit could lead to Scottish independence, a left leaning think tank has warned.

The Institute for Public Policy Research (IPPR) said the Scottish and UK governments must explore such an approach with “equal vigour”, as Scotland continues to face the prospect of being pulled out of the EU against its will.

First Minister Nicola Sturgeon has pledged to “explore all options” to keep Scotland in the EU, but said a second independence referendum is “highly likely” after Remain was backed by 62 per cent of Scots.

Prime Minister Theresa May has said she is “willing to listen to options” that the SNP present - but she has described some of the early ideas as “impracticable” and Scottish Secretary David Mundell said the idea of Scotland remaining in the UK and EU is “fanciful”.

However, the IPPR, which was highly influential on government policy during Labour’s time in government, warned that such an approach could propel Scotland towards independence.

IPPR director Russell Gunson said the Scottish Government lacks “a genuine commitment to the UK” so it is up to unionists to present options for Scotland’s EU future if they want to protect the UK.

Gunson said: “It is those across the UK who do not wish to see Scottish independence, or those open to exploring all the options, who will need to come forward with other options.”

These options “need to be explored with equal vigour and rigour by both the UK and Scottish governments”, he said.

“The latter is certainly giving the impression of being up for exploring all the options, but without a genuine commitment in the rest of the UK, and without significant reform to Britain as a whole, a tailored approach to Brexit will not be deliverable.

“And with that, Scotland’s options would be narrowed significantly.”

He added: “One of the only things we can be sure of at this stage is that the UK - including Scotland or not - will need to change fundamentally, or it will be forced to, in order to meet the multiple mandates delivered by the EU referendum result.”

An SNP spokesperson, commenting on the IPPR’s intervention, said: “As the first minister has made clear, all options are on the table to protect Scotland’s existing relationship with the EU - including another independence referendum.

“The UK Government now has a duty to show that it is serious about respecting Scotland’s decision to remain in the EU.”

The SNP also sought to highlight potential damage from Brexit, as the party cited Crown Office figures that showed more than 500 cases have been heard in Scottish courts due to the European arrest warrant.

The EU justice measure has also seen 367 people extradited from Scotland to face courts in Europe, according to figures obtained by the Scottish Parliament Information Centre (SPICe).

The SNP say these cases may not have been possible without the co-operation of fellow EU member states and urged prime minister May to maintain cross-border co-operation on justice following the Brexit vote.

SNP MSP Ben Macpherson, a member of the Scottish Parliament’s Justice Committee, said: “The EAW is a perfect example of how working together with our friends and allies in Europe helps keep us safer - and that this has been put at risk thanks to the irresponsible actions of the Tory government is simply unacceptable.”

He said the 541 cases heard in courts in Scotland as a result of the EAW “might not have been possible without the co-operation and support of other European nations”.

“That this is being put at risk demonstrates the absolute folly of Scotland possibly being dragged out of Europe against our will,” he said.

Labour MSP Jackie Baillie has written to Keith Brown, Scotland’s cabinet secretary for the economy, jobs and fair work, asking him to set out what preparations he has made, and what action he will take, to deal with the fallout of Brexit and any impact on Scotland’s economy.