NICOLA Sturgeon could bring a bill on holding a second independence referendum before Holyrood if Theresa May continues to block calls for a vote, a legal expert has said.

Sturgeon will set out how she will respond if the UK government continues to rule out plans for a second independence referendum, when MSPs return from the Easter recess a week tomorrow, April 16.

She has dismissed any suggestion of an early Holyrood election to end a stand-off with Theresa May over a second referendum. The First Minister said the scenario, alongside speculation around taking legal action to try to secure another referendum, was not one she had in mind.

The Sunday Herald spoke to number of leading experts on constitutional law to assess Sturgeon's options if Westminster continues to refuse to grant permission for a legally-binding referendum.

May has insisted that "now is not the time", and that all efforts should be on securing the best Brexit deal for the whole of the UK now Article 50, the formal process for leaving the EU has been triggered. Downing Street has yet to formally respond to Sturgeon's written formal request for a referendum between autumn next year and spring 2019.

Sturgeon, who met May in Glasgow for talks last month, has indicated she would be willing to negotiate the timing. But if May refused to agree a Section 30 order to allow a legally-binding referendum during the current Holyrood parliament, which runs until 2021, Sturgeon's hand could be forced.

Dr Tobias Lock, a constitutional law expert at Edinburgh University, said one option for Sturgeon could be to test the legal competence of the Scottish Parliament to hold a referendum by bringing a bill to hold one before Holyrood's Presiding Officer Ken Macintosh.

Lock suggested that getting a bill through scrutiny at Holyrood would pile pressure on Theresa May to allow a second referendum.

"They could call an independence referendum if they can get it through the Scottish Parliament and it's not thrown out by the Presiding Officer," said Lock.

In early 2010 Alex Salmond's minority government published a draft bill on a proposed referendum on independence, in what was seen as an audacious challenge by Sturgeon's predecessor to the Unionist parties.

Salmond's government later withdrew the bill after failing to secure opposition support.

But the current pro-independence majority at Holyrood, when SNP and Green MSPs combine, would mean a bill from Sturgeon would be passed.

Any bill would have to go through three parliamentary stages, including votes by MSPs and committee scrutiny.

The move would allow Sturgeon to lay down a fresh challenge to Westminster of having secured democratic legitimacy from MSPs with the bill, on top of Holyrood's vote in favour last month.

Sturgeon would almost certainly be able to do this within a timescale that would allow a referendum to be held by spring 2019.

Lock also suggested Sturgeon could hold a non-binding advisory vote of the electorate asking it to confirm Holyrood’s wishes for a referendum.

However, Lock warned opponents could boycott such a vote, in order to "delegitimise" it as happened in the 1973 border poll held on Northern Ireland's place in the UK, when less then 1 per cent of Catholics reportedly voted.

Lock said: "That would make it harder for Westminster to reject it. Technically I don't see why not, as it's just a request for another Section 30 order.

"It could be asking the Scottish people to affirm what the Scottish Parliament has already requested. Whether that's a wise move is another thing, but legally they could.

"That seems to be one option, but you could get the problem that one side boycotts it as happened in the border poll in Northern Ireland some decades ago. You could get a similar reaction from Unionist parties here asking people not to turn out so as to delegitimise it."

Constitutional expert Professor Matt Qvortrup, said May's refusal to allow one before Brexit could hand Sturgeon the option of having the vote on the same day as the 2021 Holyrood elections and winning it by making the Tories look undemocratic.

Qvortrup, author of the book Referendums Around The World, said that delaying a referendum could suit the SNP and make May appear increasingly intransigent in the eyes of Scots.

He also suggested that another tactic could include the "Catalan option" of having a referendum without Westminster permission.

The academic said another possibility would be to launch a citizen-led petition to demand a referendum – something he pointed out was used in the US state of Alaska, as well as in parts of Germany and Latvia.

He said: "One alternative is the option that is used in Alaska where people can demand a referendum if they get a certain number of signatories. They have this in Latvia as well and in most German states." In the UK the Westminster Parliament also has to debate a motion if a petition has more than 100,000 signatories.

Qvortrup continued, "But the chance of getting independence is greater if there's a bad Brexit deal.

"There's the Catalan strategy of saying we're just going to have a vote anyway. But Theresa May would throw the book at her [Sturgeon].

"The best option is to appear very miffed and keep saying how undemocratic it is that for two years Scotland will not be allowed a referendum by these bureaucrats who have no mandate in Scotland. The climate for a good deal is limited.

"I'd say that come 2020-21 the best option would be to say Scotland has suffered and and lay all the blame for this on Theresa May who they would say has not been democratic. They could probably have a referendum in May 2021 on the same day as the election."

However, one of the UK’s leading constitutional authorities, Vernon Bogdanor, said that whatever Sturgeon decided, ultimately the SNP would be unable to act if May refused a second referendum because the power is reserved to Westminster.

Bogdanor, a visiting professor of political history at Gresham college in London, said: "I can't see what the Scottish Government could do if the UK Government said no to a referendum.

"There's been talk that they could hold their own referendum, but they could face a legal challenge for doing something that's ultra vires if they were to spend public money on it.

"They could get a petition asking that the power to hold a referendum be transferred and it might put pressure on the UK Government. Ultimately the power to call a referendum is a UK government power and not a Scottish government power."