THERESA May is “the block” to getting a deal with Nicola Sturgeon on the Brexit Bill, say the SNP leadership, who have praised her deputy for better understanding the principles of devolution.

As the Prime Minister today kicks off her four-nation whistle-stop tour in Scotland to mark a year until Britain’s withdrawal from the EU, the Nationalists also said they were “baffled” by her upbeat claim that London and Edinburgh were edging closer towards a deal on the flagship legislation when, they insisted, the reality was that the talks remained deadlocked.

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The remarks from Ian Blackford, the SNP leader at Westminster, came as a report from top pollster Sir John Curtice claimed most people in the UK would soon oppose Brexit because of “relentless” demographic changes.

Published by The UK in a Changing Europe think-tank, it estimated the 52-48 vote for Leave in 2016 would be reversed by 2021, becoming 52-48 for Remain, and reaching 54-46 in favour of Remain by 2026.

Today, Mrs May will begin her journey around the UK from Ayrshire, vowing to deliver a Brexit that “unites the United Kingdom”.

But her flagship EU Withdrawal Bill is still dividing her government from the administrations in Edinburgh and Cardiff.

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The First Minister and her Welsh counterpart, Carwyn Jones, claim the legislation is a “power-grab” as the PM wants a temporary pause on some powers covering agriculture and the environment transferring from Brussels so that common frameworks can be agreed to protect Britain’s internal market.

Just 24 hours after the PM told MPs on Tuesday evening that the UK and Scottish Governments were “edging closer and closer” to an agreement on the bill, Mr Blackford expressed consternation at her upbeat assessment.

“I can’t for the life of me think why the PM can make such a statement because it’s not based on the reality of what is happening on the ground. It baffles me,” declared the Highland MP.

“The PM is coming up to Scotland with a year to leaving the EU and the fact we can’t get a reasoned agreement between London and the devolved nations is not great. The block to that is the Prime Minister, I’ll be that specific…She is driving this.

“It’s Mrs May who is not recognising she is standing in the face of the Scotland Act 1998 that is very clear about what are devolved and what are reserved matters…”

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Mr Blackford claimed David Lidington, the PM’s de facto deputy, who has been leading for Whitehall in the intergovernmental talks on the bill, “does understand the principles of devolution,” but, he argued, the PM did not.

Ahead of today’s countrywide tour, Mrs May hailed the UK as the “world’s most successful Union,” insisting it was an “enormous force for good".

She said: “Make no mistake, this government is absolutely committed to the devolution settlements as we have demonstrated beyond question with landmark pieces of legislation over the last few years.

“But as the Prime Minister of the United Kingdom, I have an absolute responsibility to protect the integrity of the United Kingdom as a whole; that means ensuring no new barriers are created within our common domestic market and that the UK is able to meet its international obligations in the future.

“No Prime Minister could leave these things to chance because they are absolutely crucial to our success as a country in the future,” explained Mrs May.

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Her trip will involve meeting workers at a textile factory in Ayrshire, visiting a parent and toddler group in Newcastle, lunching with farmers near Belfast, talking to business leaders in Barry and ending the day taking tea in west London with a group of Polish citizens who have made Britain their home.

To coincide with the think-tank’s report, a conference will be held in London with guest speakers including Sir John, Professor of Politics at Strathclyde University, and Nigel Farage, the former Ukip leader.