Theresa May will vow never to allow the UK to become "looser and weaker'' in a speech ahead of a meeting in Scotland with Nicola Sturgeon.
UKIP Wins First Scottish MEP
UKIP has won its first parliamentary seat in Scotland after a knife-edge European election contest.
David Coburn won the seat for the anti-EU party - which dominated in England - after picking up 10.4% of the vote.
The result was delayed until today to allow for counting in the Western Isles.
It caps a remarkable European election for UKIP, which saw it stretch ahead of Labour and the Conservatives with the highest share of the vote across Britain as a whole.
In Scotland, the SNP and Labour remain on two seats each while the Tories keep their single seat.
The Lib Dems were beaten into sixth place behind the Greens and lost their only Scottish seat in a dreadful UK-wide result for Nick Clegg's party.
Scottish Lib Dem leader Willie Rennie described the results as "heartbreaking".
The complicated proportional voting system meant that the SNP had to extend its share of the vote beyond one-third of the total in Scotland if it had any chance of keeping UKIP out.
But Alex Salmond's party fell just short with 389,503 votes - a 28.9% share.
The Green Party had also hoped to pick up its first seat and block UKIP, but ended with 108,305 vote (8%).
Labour saw its vote increase by five points to 25.9% with 348,219 votes, keeping its two MEPs, while the Conservatives held on to their single seat with 231,330 votes (17.2%).
The Liberal Democrats won just 95,319 votes (7.1%).
Turnout in Scotland was just 33.5% - which is still an improvement on 2009 when only about 28.5% of the electorate voted.
Across Scotland, turnout peaked at 43.1% in East Renfrewshire and 41.6% in Edinburgh. It was lowest in North Lanarkshire at 28.1%.
UKIP's winning candidate, David Coburn, said: "The UKIP revolution has now come to Scotland.
"I will do my best to make sure I highlight the problems of the European Union. When I'm in Brussels I will do my best to make sure Scottish business and Scottish people know the daft schemes they're cooking up there to make our lives infinitely more awkward.
"My second mission will be to make myself redundant. I want to get out of the European Union at the first available opportunity, and that I will do.
"On the bigger picture, Ukip has done well on the British stage as a whole and the fact that we're winning in Scotland and got such a good score in Scotland - getting up to 14% in some areas - is something we're very proud of."
Mr Coburn said he will fight hard against the campaign for Scottish independence.
He pledged to ensure every Scottish soldier serving abroad gets a vote in the referendum.
Of the six winners, only Mr Coburn does not live in Scotland. His address was declared as being in Kensington, London.
Ian Hudghton, re-elected for the SNP, said his party won overall.
"We also have the opportunity in September to ensure that we transform our experience as members of the European Parliament, and as a country in the European Union, by voting 'yes' and becoming a normal independent member state with the right to representation as a nation, as a government, as well," he said.
The Scottish independence referendum will be held on September 18.
An analysis of the European votes suggests the pro-independence parties - SNP and Green - secured about 37% of the vote, on a low turnout across Scotland.
Returning Labour MEP David Martin said SNP leader Alex Salmond has allowed UKIP in by polarising the debate in Scotland.
"I'd like to suggest to David Coburn that the first thing he does when he sits down and recovers from the champagne is write a letter to Alex Salmond because there is no doubt in my mind that Alex Salmond's decision to polarise the campaign, the Scottish debate, between two extremes is why David Coburn is with us today as a member of the European Parliament," he said.
Mr Martin, flanked by Labour's other returning MEP Catherine Stihler, said: "We knew before we set out on this campaign that many people up and down the country are suffering real hardship under the present policies.
"Catherine and I are determined to use the platform that Scottish people have given us in the European Parliament to continue to campaign for Scottish jobs, to continue to campaign for social justice, to continue to work for fairness at work, and we're committed to work hard for all the people of Scotland."
He offered his commiserations to outgoing Lib Dem George Lyon.
"I'm particularly sorry for George Lyon," he said.
"George was a very conscientious member of the European Parliament and his demise reflects the demise of his party, not his own efforts inside the European Parliament."
Tory MEP Ian Duncan, who takes over from the retired Struan Stevenson, said his party's vote is the highest it has been for 25 years.
"We've bucked the national trend and it is a reminder that we have much to offer the people of Scotland," he said.
"People are concerned about Europe and they want change. They want change to make Europe work for them, not against them, and I believe we have harnessed that concern and that feeling in a very simple way."
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