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.
Scottish Labour Leaders Election Begins
Voting opens in the Scottish Labour leadership election today with candidates Kezia Dugdale and Ken Macintosh both pledging to change the party.
Ms Dugdale says she has the support of the overwhelming majority of local parties, trade unions and elected representatives, but Mr Macintosh is appealing directly to members to use their new voting power to "choose a new path'' from those currently running the party.
Scottish Labour scrapped its tripartite electoral college system, which allowed trade unions and parliamentarians to overrule elected members regardless of the size of the popular vote, in the wake of its crushing defeat in the general election in May.
The next leader will be elected under a one member one vote system, giving grassroots members an unprecedented say in the future of the party.
Mr Macintosh said: "From speaking to members, it is clear that the vast majority are still undecided in this contest.
"From Monday, my team of volunteers will be hitting the phones to get the message of change across. I'm writing out to all members and supporters and have produced a video which I will also be emailing to all those who have signed up online.
"My whole appeal is to the members rather than to those who run the Labour Party.''
He added: "Under my leadership, Scottish Labour will be driven by our positive vision for Scotland's future and not by our opposition to the SNP.
"For the next three and a half weeks, the focus will be on making sure members know the future of the party lies in their hands, that it is their decision that counts. I will be urging them to choose a new path.''
Ms Dugdale said she is "reaching out to every corner of the party's membership to deliver a mandate for the change our party needs''.
"It's clear we need TO start promoting and supporting a new generation of Scots who want to see an end to injustice, and to give them the space and support needed to call the Scottish Labour Party home,'' she said.
"Labour has always succeeded when it's been the party of the future, offering a space for talented and passionate people to come together and deliver meaningful change. Under my leadership it will be again.''
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