A LEADING critic of having a Plan B route to independence has criticised the SNP for rejecting a motion calling for a back-up strategy should Boris Johnson continue to refuse a referendum.

Pete Wishart, the SNP's longest serving MP, retweeted an article from The National reporting the rejection by the party to include the motion at its conference next month.

He wrote this morning: "This is a mistake. We need this damaging debate concluded so we can unite around an agreed way froward."

The Perth and North Perthshire MP is regarded as one of the most vocal critics of Plan B.

In June he said a proposed route to independence which would avoid a Westminster veto should be "put to bed" - amid fears it could force Scotland into a "hellish limbo" like Catalonia.

He argued that such a route to independence would in effect mean declaring "some sort of Unilateral Declaration of Independence".

READ MORE: Scottish independence: 'Plan B' motion rejected for SNP conference vote

"My little bit of advice to my good friends in the ‘Plan B movement’ is to at least come up with some sort of concrete proposal so we can have some idea what it is we are supposed to debate," he wrote on his blog.

"Just 'doing it anyway' means we would be doing something broadly similar to what Catalonia did when they ‘won’ their uncontested referendum – without actually winning a referendum! This would in effect mean we would be declaring some sort of Unilateral Declaration of Independence (UDI).

"The consequences of that could not be more serious. Almost certain to be one of the first things to happen is that we would have all of this immediately ruled illegal and be disenfranchised from the entire international community. We would be left in the sort of hellish limbo currently endured by the people of Catalonia."

A proposed motion by Angus MacNeil MP and councillor Chris McEleny had been put to the SNP's conference committee which draws up the event's agenda.

They failed to get their resolution on the agenda last year but tried again hoping a second attempt would be successful.

First Minister and SNP leader Nicola Sturgeon is wary of any Plan B route, and wants an approach which would follow the process behind the 2014 vote.

She has also cautioned activists against straying from this model, warning that it could lead to a result which was not recognised by the UK or the wider international community.

The SNP's online conference is being held from November 28 to 30.