JOANNA Cherry has hit back at Alyn Smith, insisting she is not advocating an "illegal referendum" after calling for Holyrood to legislate for a second independence vote.

The SNP's justice and home affairs spokeswoman at Westminster set out her plan in a lecture on Friday about what would be a way forward if Boris Johnson continued to block an agreed plebiscite between the UK and Scottish governments if there is a pro-independence majority in the Scottish Parliament after the election next May.

Johnson has repeatedly said he will not agree to a second vote saying the independence side described the 2014 referendum as a "once in a generation event".

Cherry, a QC, said the Scottish Parliament could introduce a bill to stage a vote which it inevitably meeting a legal challenge, arguing there would be nothing to lose for the Yes side by doing so.

However, asked about Cherry's proposal Smith compared it to the referendum in Catalonia where voters backed independence but because it had not been recognised by Spain had not delivered the result. He described her plan as a "gesture".

READ MORE: Alyn Smith and Joanna Cherry clash over call for Holyrood to hold indyref2

Cherry, MP for Edinburgh South West, responded today citing report in The National on Smith's comments, which he had made on BBC Radio Four's Any Questions yesterday and Friday evening.

"What a shame ⁦Alyn⁩ didn’t read my speech before commenting. I supplied a copy to the party in advance. It would be a little surprising if a senior lawyer was advocating an “illegal” referendum. Time to get our ducks in a row," she wrote.

In her lecture Cherry referred to comments made by Nicola Sturgeon in her Brexit Day speech in January when she drew attention to a possibility of Holyrood bringing in a bill which would enable the Scottish Government to stage a second independence referendum with the First Minister warning it could “move us forward – but equally it could set us back”.

But Cherry disagreed. “It is my view that if the pro-independence referendum parties obtain a majority at the Scottish election next year and the PM refuses to come to the table to negotiate a second Edinburgh Agreement, the avenue which the FM contemplated earlier this year should be pursued,” she said, delivering the Wales Centre for Government’s annual lecture.

She insisted that whatever the outcome, the independence cause would not be damaged and underlined the political context in which any future referendum bid was made.

"It would be for the courts to decide whether the bill passed was within the competence of the Scottish Parliament and, thus, whether the referendum so authorised could proceed ... The case would undoubtedly end up in the UK Supreme Court," she said.

"If they found the bill to be within competence, then we would have a lawful referendum. And one which would be hard for unionists to boycott. If we lost then I do not believe we would be any further back than the stalemate that will ensue if Boris Johnson digs his heels in.”

Asked whether he agreed with Cherry about Holyrood holding an independence referendum if the PM continued to block a new vote, Smith, the SNP foreign affairs spokesman in Westminster and a former MEP, made clear he did not.

The Stirling MP said: “No I don’t. I don’t believe in gestures like that. I think we’ve seen in Catalonia where that leads to. I am here to defend party policy. The party’s policy is we want an agreed referendum with the UK as we did in 2014.”

Pressed that the Scottish Government “wasn’t going to get one” as Johnson had said he won’t agree to one, Smith added: “That position is utterly unsustainable in the eyes of the people of Scotland.

“And every time he does say no we go up in the polls so that is neither democratic or sustainable and that is a political argument.”

He added: “We are a constitutional party we want to join the international legal order as an independent state and that needs to be legitimate in the eyes of the people of Scotland and the international community. So yes of course there are other things we can look at, there are other things we will look at as things go forward.”

It is the latest clash between Smith and Cherry.

Last week the Stirling MP criticised the SNP Common Weal Group (SNP CWG) for launching a list of candidates running for election to the party’s ruling NEC. The SNP CWG are backing candidates – including Cherry – who signed its manifesto calling for greater internal democracy in the party.

But Smith argued there should be "no parties within parties".