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.
Independence: One Month To Decide
Independence polls have narrowed as more undecided voters make up their minds with just one month until Scotland goes to the ballot box.
A total of 38% of people said they support a Yes vote in the latest poll by ICM, up 4% from its poll last month.
Support for a No vote also picked up 2%, increasing to 47% in the August survey, which was carried out for the Scotland on Sunday.
When undecided voters are factored out support stands at 55% for No and 45% for Yes, a 2% move towards Yes from last month, ICM found.
The poll of more than 1,000 people was carried out after the first TV debate between First Minister Alex Salmond and Better Together leader Alistair Darling and 42% said Mr Darling came out on top with 19% believing Mr Salmond won.
Arguments over currency dominated a large part of the debate and the ICM poll found 52% of those surveyed said they thought the Scottish Government's currency plans were "unconvincing''.
The Scottish Government favours a formal currency union with the rest of the UK which would allow an independent Scotland to continue to use the Bank of England as its central bank.
The UK Government and the main Westminster parties have already ruled out that option.
Despite the TV debate findings, the Better Together campaign has not gained any long-term advantage, a polling expert said.
Writing on the What Scotland Thinks website, polling expert Professor John Curtice, of Strathclyde University, said: "Today's poll puts Yes on 38%, up four points on a month ago.
"No are on 47%, also up on last month but by a more modest two points. Don't Knows (which have tended to be relatively high in ICM's polls) have fallen by no less than seven points from 21% to 14%, suggesting that undecided voters have indeed begun to make up their mind as polling day approaches.
"Once the Don't Knows are excluded the Yes tally stands at 45%, No at 55%, representing a two point swing from No to Yes since last month. A movement of that size is too small for us to be able to rule out the possibility that it has simply occurred by chance, and in any event it simply puts the Yes vote back to where it was in ICM's poll in June.
"Today's poll is thus best regarded as failing to show that No has gained any longer-term advantage from the leaders' debate than as evidence of a renewed swing to Yes.''
Another poll by Panelbase, commissioned by the Yes campaign, had similar findings. Carried out over the same period and among similar numbers to the ICM research, it found that 42% support a Yes vote with 46% for No and 12% undecided.
When the undecided voters are taken out it stands at 48% for Yes and 52% for No, a 2% swing to Yes from the last Panelbase poll.
Deputy First Minister Nicola Sturgeon said: "These are extremely encouraging findings, showing rising support for Yes and putting the referendum on a knife edge.
"We have a lot of work to do, but the polls show a two point rise in support for Yes in the last couple of weeks and Yes only needs a 2% swing to move ahead - I am confident that we can and will achieve this in the month ahead.''
Better Together campaign director Blair McDougall said: "These two opinion polls show that the majority of Scots are continuing to reject separation as Alex Salmond still cannot outline his currency Plan B.
"In one week up to one million Scots will start casting their referendum vote by post but Alex Salmond and the Nationalists can't answer the basic questions, such as what currency would be in our pocket or how our public services would be funded.
"The currency issue matters. Scots want to know if their job will be safe, if their pension will be safe and how much their shopping and electricity bills would cost. Alex Salmond can't ease those fears if he can't even tell us what currency would be in our pocket.
"The ICM poll shows that undecided voters, when pressed, are saying 'No Thanks' to Alex Salmond's separation plans. That's incredibly encouraging for our campaign. Alex Salmond has had two years to set out his case for breaking up the United Kingdom but can't even tell us what the currency would be.''
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