Nicola Sturgeon will miss a spring deadline to hold an independence referendum in time to keep Scotland in the European Union, an expert has warned.

The SNP would have to call, fight and win a vote by early 2017 to beat Prime Minister Theresa May's Brexit timetable, says veteran Brussels watcher Kirsty Hughes.

However, the first minister and her party have run out of time to prepare the political and parliamentary ground for an early referendum in the coming year as they weigh up what Ms Hughes describes as rapidly narrowing options.

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Polls show support for independence remains short of the clear majority analysts claim Ms Sturgeon would want to see before calling an historic second vote.

But Ms Hughes - an authority on EU politics formerly of Chatham House - argues that there is a simple schedule Scotland would have to meet if it wants to seamlessly stay in the bloc.

Writing for Edinburgh University's Europa Institute, she said: "The timescale now looks very tight for those who would like a second independence referendum before the UK leaves the EU, so that Scotland can simply stay in when rUK leaves.

"If Scotland would need 18 months to two years to disentangle from rUK if there was a Yes vote then, unless an second referendum is held in the first half of 2017, it is unavoidable that Scotland will leave the EU when the UK does."

She added: "A referendum by the end of 2018, would mean Scotland could, as part of its divorce talks with the UK, hold parallel membership talks with the EU, and attempt to limit the damage so that its laws did remain consistent with the EU, allowing fast-track negotiations and re-entry.

Read more: MPs must have vote before Brexit process begins to ensure UK Government has "clear plan"

"But it would not be anything like as smooth or speedy as could be achieved if there was a very rapid referendum in early 2017, which currently looks unlikely."

However, Ms Hughes warns that referendum in the early 2020s - after Brexit - could lock Scotland out of the bloc for years. Delays, she explained, "could see a greater divergence between Scottish and EU laws and regulations in the meantime and a longer, slower route back into the EU – if that remained Scotland’s goal – perhaps by 2030".

Ms Hughes - long before June's Brexit vote - had highlighted complex opportunities for Scotland to retain some of the benefits of EU membership. She said the UK Government did not look likely to agree to these.

She said: "Scotland will have no protection from the negative economic effects of Brexit in the absence of any major differentiation in its relationship with the EU compared to rUK."

Prof Jo Shaw of Edinburgh University agrees that options for "differentiation" had been closed down by the current UK government. She said: "It’s natural for eyes to turn back to the question of when and a second referendum might be held, and how that might work out.

"Ms Hughes suggests that the timescales are very difficult indeed for the Scottish Government, giving little room for manoeuvre in terms of preserving membership even if the referendum is won.

Read more: MPs must have vote before Brexit process begins to ensure UK Government has "clear plan"

"There would be likely to be a difficult transitional period during which much of the economic damage which Brexit is predicted to cause to the UK economy may already be done."

Prof Shaw, who is also an expert in EU institutions, suggested Brexit may change Scottish views on the bloc.

She said "Whatever Scottish government is in power if that question were to be raised in the future might decide that a Norway/Iceland style approach more fits a self-conception of Scotland as quasi-Nordic."

The Scottish Government did not comment on Ms Hughes's warnings on timings. A spokeswoman said: "Our priority is to protect all Scotland’s interests. We are considering all options to ensure Scotland’s continuing relationship with, and place in, the EU.

"It is essential that the UK Government start having meaningful discussions with the devolved administrations and to answer basic questions such as the UK’s future in the single market.

"If we find our interests can't be protected in a UK context, independence is an option and people in Scotland must have the right to consider it."