Mike Russell, the Scottish Government’s Constitution Secretary, will present the 11-point document to the party’s policy forum on Sunday.

It says a “legal referendum” will be held after the pandemic if there is a pro-independence majority following May’s election.

The roadmap states any attempt by the UK Government to challenge the legality of the referendum in the courts will be “vigorously opposed”.

Mike Russell, the Scottish Government’s Constitution Secretary, will present the 11-point document to the party’s policy forum today Sunday.

Russell says the UK Government could either agree that Holyrood already has the power to hold a second referendum or agree to a Section 30 order – something he said would put the question of legality “beyond any doubt”.

Boris Johnson has repeatedly stated his opposition to a second independence referendum.

As the roadmap document was published on Saturday, Russell said: “I firmly believe that Scotland’s referendum must be beyond legal challenge to ensure legitimacy and acceptance at home and abroad.

“The referendum should be held after the pandemic, at a time to be decided by the democratically elected Scottish Parliament. The SNP believes that should be in the early part of the new term.”

He continued: “Today I am setting out how I believe that right can be secured, and I welcome the discussion that will take place around this idea and others.

“But what is absolutely not for discussion is the fact that if Scotland votes for a legal referendum on May 6 this year, that is what it will get.

“The SNP Scottish Government will deliver such a referendum if re-elected and the proposals I am putting forward make that very clear.”

Around 1000 party members are expected to take part in the SNP’s national assembly tomorrow, a policy forum chaired by deputy leader Keith Brown.

Opposition parties accused the SNP of putting the push for independence ahead of the coronavirus pandemic.

“It is inexcusable that at this time of acute crisis the SNP seeks to put its plan for independence above everything else.

“The people of Scotland are being badly let down by an incompetent UK Government and a Scottish Government that seeks to exploit the current crisis for its own ends.

“To turn your back on those most in need by banging the drum for another independence referendum is an act of political hubris and is truly revealing of the Scottish Government’s true priorities.”

Meanwhile, responding to Mike Russell’s proposals for a road map for another independence referendum, Scottish Liberal Democrat leader Willie Rennie said:  “The SNP have got their priorities all wrong. We need a needle-sharp focus on putting recovery first, not an endless focus on constitutional issues that squeezes out the things that really matter.

“Thousands of people have lost their lives, even more are living with the long term effects of the virus, the hospitals are bursting at the seams, operations are cancelled, businesses are on their knees, schools are shut yet the SNP think now is the time to restart their campaign to break up the UK. It is crass, insensitive and the wrong priority for our country.

“The SNP seem determined to repeat the mistakes of Brexit with the Brexit II sequel. It’s about time the SNP woke up.”

Scottish Conservative leader Douglas Ross tweeted: “When 100% of our focus should be on recovering from the pandemic, the SNP are charging ahead with plans for another referendum.

“We won’t let them get their way.”