NICOLA Sturgeon is today called on to “come clean” and “be honest with the people of Scotland” on whether or not she will include a full-blown commitment in the SNP’s 2016 Holyrood manifesto to holding a second independence referendum in the next five years.

In throwing down the challenge to the First Minister, David Mundell also signalled that if there were just a form of words in the Nationalists’ policy programme to allow “maximum flexibility”, then this would not be a mandate to hold another poll, suggesting that, in those circumstances, Westminster – which remains the constitutional authority for holding legal referendums – would not sanction one.

In an exclusive interview with The Herald, the Scottish Secretary addressed growing speculation that Ms Sturgeon will use her party’s annual conference in October to announce her determination to keep open the option of a second referendum in the next Holyrood session.

Mr Mundell, Scotland’s only Conservative MP, stressed that the First Minister in the referendum campaign announced that it was a once in a lifetime vote and that in the General Election campaign that there were no plans for another poll.

“They have made it clear to people who voted for them that there wasn’t going to be another referendum and if they changed their position and were now supporting having another referendum, then people in Scotland would be questioning their credibility and their commitment to focus on Scotland rather than just on a constitutional obsession,” declared Mr Mundell.

The Secretary of State was pressed on what the UK Government would do if the SNP leader sought a popular mandate for another referendum at the Holyrood elections and got one. Would Westminster facilitate or block a second poll?

“I'm proceeding on the basis Nicola Sturgeon has said there isn't going to be one; that's what she said ahead of the UK General Election...So the challenge is for her and the SNP to come clean on where they stand in relation to making having another referendum the focal point of next year's election.”

Following the General Election David Cameron insisted the issue of Scottish independence had been “settled,” stressing: “There isn’t going to be another referendum.”

The SNP leader reacted forcefully, arguing the Prime Minister had “no right” to make such an assertion. Indeed, Alex Salmond, her predecessor, suggested if Westminster blocked the way to a legal second referendum, Holyrood would simply hold one regardless.

Ms Sturgeon has previously said there would need to be a “material change” for a second referendum to be held, citing the only example as Scotland voting to stay in the EU in the forthcoming in/out poll while England voted to come out.

But Mr Mundell said: "Using a wording that allows maximum flexibility would hardly be capable of being held out as a mandate for a second referendum.

“If you want a second referendum, be upfront and be honest with the people of Scotland that that is your priority ahead of how much tax they pay, what welfare benefits they get, what happens in our schools or hospitals. It's time to be honest about it.

“If there is going to be a referendum commitment, let's not be mealy-mouthed about it and hide it away in some ifs and buts. Let's be honest with the people of Scotland about it and then we will all know where we stand," he added.

But an SNP spokesman said: "It is David Mundell who has not been honest; in the last desperate days of the referendum, the No campaign promised the people of Scotland powers for the Scottish Parliament near to federalism but the Scotland Bill falls far short of that.”

He noted how every single Bill amendment for more powers - even when voted for by 58 of Scotland's 59 MPs - had been vetoed by the Scottish Secretary.

"The timing of any future referendum is entirely a matter for the people of Scotland to decide; the people, not politicians, are in charge at every stage of the process.”

He added: “The First Minister has made clear we are not planning another referendum but equally has made it clear that it is not in the gift of any politician and party to rule it out indefinitely."