THERESA May has promised to lead an intensely “Unionist government” if returned as Prime Minister next month, as she claimed a vote for any party but the Tories would weaken the UK.

In a speech aimed at Labour swing voters, Mrs May said it was time to leave the “old, tribal politics” behind and “come together in national interest” to make Brexit and the UK work.

Speaking at the launch of the Scottish Conservative manifesto in Edinburgh, she said: “Come with me as I lead Britain. Strengthen my hand as I fight to strengthen our Union.”

She went on: “That stronger Union is a personal priority for me. As long as I am Prime Minister, I will never stand by and let our Union drift apart.”

However Mr May’s new refusal to allow a second independence referendum until there is “public consent” and the Brexit process has “played out” came under heavy fire after she and senior Scottish colleagues repeatedly failed to define what the terms meant.

Former First Minister Alex Salmond said the Tories were “mired in confusion” on the issue.

As a YouGov poll suggested the Tories could gain eight SNP seats on June 8, Mrs May said the SNP’s “obsession” with independence had led to a decline in public services.

She said: “After a decade of SNP neglect, standards in Scottish education - which were once the envy of the civilised world - have become a national scandal.

"The SNP's tunnel-vision obsession with independence above everything else has meant young people in Scotland are being let down. Education might be a devolved matter - but I still care very deeply about the life chances of Scottish children.

"As Prime Minister of the United Kingdom, I care just as much about the futures of children from Ayrshire and Angus as I do about the futures of children in Kent or Carlisle.

"I want everyone in our country to have the chance to live secure and prosperous lives, and reach full potential. That is because of a simple truth. Across the United Kingdom, we may be four nations, but at heart we are one people. We all have a stake in each other's success."

In a message aimed at the SNP-Tory marginals in the Northeast, she also said the SNP wanted to take Scotland back into the EU and the Common Fisheries Policy.

“A vote for me is a vote for a better future for fishing,” she said. “A vote for the SNP is a vote for the Common Fisheries Policy.”

Mrs May was echoed by Scottish Tory leader Ruth Davidson, who called for a “reset” of the Curriculum for Excellence system in schools.

She appealed directly to Labour voters, insisting the Tories were now “mainstream” and “rooted to the centre ground”, and only they could “lead the fightback against the SNP”.

After the two leaders gave their speeches, they were pushed on how they would measure when public consent was right for a referendum and when Brexit would play out.

The Prime Minister twice refused twice to be drawn on a definition of public consent.

She said: “Talking about a second independence referendum at this time is trying to pull us apart, just at the very time that as a nation.. we should be pulling together."

Asked whether he was setting deliberately vague tests to postpone a referendum indefinitely, Scottish Secretary David Mundell said: “We’re not setting a definitive set of criteria. What we’re saying is that public consent has to be apparent. It’s not a con trick at all.

“A majority in the Scottish Parliament doesn't count in the simple sense because having a referendum is an agreement between the UK Government and the Scottish Government."

He said "sustained support" from the public would be needed, but asked to define this replied: "Well, we’re just not getting into that."

Asked how long he thought it would take Brexit to “play out”, he said there would need to be a transition and implementation phase after spring 2019 which could last for several years.

“Then, if the Nationalists want to continue to push independence, we will argue there shouldn’t be a referendum unless there is public consent for that,” he said.

Asked if he foresaw the process being complete before the next Holyrood election, he said: “I don’t know the timescale. The electoral cycle isn’t going to necessarily fit with how the process unfolds. I don’t know what stage we’ll be at when we reach the 2021 Holyrood election.”

Asked her definition of public consent, Ms Davidson said: "The principle is you cannot ask people to make a decision about the constitutional future of our country when you don't know what that future looks like. You don't know what Brexit looks like.. and you don't know what independence looks like because they haven't outlined that either.

"This isn't about timings, dates, parliaments, whatever, this is about the principle.”

She said that in 2011/12, before the last independence referendum, every MSP had backed it, as did more than 90 per cent of the public, adding: "We need something a lot more like what we had in 2011/12 before I think the UK Government should give its consent."

Scottish Labour leader Kezia Dugdale said: "Theresa May said that a vote for her is a vote for a better future, but the reality is that a vote for the Tories is an endorsement of a hard Brexit and the horrific rape clause.

"Rather than fix the problems they have created in our country, the Conservatives are intent on continuing to divide our country and increase the risk of a second independence referendum."

Scottish LibDem leader Willie Rennie said: “Their cuts to pensions, social care and free school meals shows that the Conservative Party is mean spirited and cold hearted. It is lurching to the right to capture votes from Ukip. The Conservative-UKIP pursuit of a damaging hard Brexit will be damaging to our security, jobs and the NHS."