Nicola Sturgeon has failed to categorically rule out unilaterally holding an advisory independence referendum after claiming she had “various options” if Theresa May refuses to give her the powers to hold a legal vote.

The First Minister’s spin doctors attempted to kill off speculation that she was considering the plan after Angus Robertson, the SNP’s Westminster leader, promised unequivocally that another vote would happen on her timetable.

Her official spokesman insisted she did not need a plan B for an advisory referendum as she had a democratic mandate for her proposals and there was no “intention” to call an advisory vote.

However, her stance became shrouded in confusion when she gave a TV interview indicating she did have a back-up plan. Ms Sturgeon said she would consider “various options” but declined to disclose what they may be.

However, they would be expected to include mass protests, marches and petitions. Ms Sturgeon’s spokesman rejected holding a snap Holyrood election, arguing it would be pointless as Mrs May could continue to ignore any fresh SNP mandate.

If they tried to hold an advisory vote, the Nationalists would face the embarrassing prospect of Ken Macintosh, Holyrood’s presiding officer, blocking the tabling of a referendum bill because constitutional affairs are reserved to Westminster.