BOTH sides of the independence debate will attempt to win over undecided voters this week with the launch of poster campaigns as senior Coalition sources admitted they have to have a more "explicitly" positive pro-Union message.

A new opinion poll published yesterday suggested the SNP are within touching distance of winning the referendum and a swing of just 2% is required to secure a Yes vote in September's referendum.

The survey, by ICM, put the No vote at 42% with Yes at 39%. The main change was a drop in backing for a No, which fell from 46 to 42% in a month, while support for Yes remained steady. Another 19% said that they did not know how they would vote.

Steven Camley's cartoon

Last night First Minster Alex Salmond MSP said the poll showed that the Yes campaign had momentum "because it is more positive and more trusted than the No campaign". "In contrast the No campaign is in a panic because they are seen as negative and unbelievable," he added.

The Tory-LibDem Government rejected accusations that its campaign is on the wrong track or scaremongering but a government source said: "We need to be more explicit about the fact that we are being positive in our message.

"Most of what we say is very positive, but it is the negative parts that are currently attracting attention. We need to be more overt about how positive we are actually being."

He added that many recent interventions by Coalition ministers had been "overwhelmingly" positive in tone, but that was not the message that was getting through.

Better Together's advertising campaign carries their new slogans "No thanks" and "Best of both worlds". Five separate versions of the poster are due to be released in the run up to polling day.

Meanwhile, a new Yes Scotland poster proclaims CAN - the image having been amended from CAN'T - and is designed to show the potential of an independent Scotland. Another advert, with the words SHOULD and MUST says that as many as 100,000 more children in Scotland could be pushed into poverty as a result of the UK Government's welfare cuts.

The images will appear on billboards and in cinemas across the country until September 18.

Yes Scotland is said to have been boosted by donations worth at least £3 million from Colin and Chris Weir, who won more than £161m on the Euromillions in 2011. Analysis by a Sunday newspaper said that the sum of money given made the couple, from Largs, Britain's biggest political donors in the last 15 months.

The new advertising drive by both sides follows a difficult week for the pro-Union campaign.

Last week Defence Secretary Philip Hammond was accused of "emotional blackmail" after he warned thousands of jobs would be at risk if Scots voted for independence. In the most serious split yet in the pro-Union campaign, Labour sources turned on the Tory cabinet minister, accusing him of "unhelpful" negative attacks on the SNP and of gatecrashing a visit by his shadow number to Scotland.

There was confusion after Mr Hammond said everything would be up for discussion if Scots voted for independence - although aides later insisted that did not include a proposed currency union which has been rejected by Labour, the Tories and the Liberal Democrats.

The SNP have long accused the No campaign of scaremongering over the effects of independence.

But pro-Union sources in turn accuse the nationalists of hypocrisy - saying that they constantly talk down the successes of the UK in the most negative of ways.

Last night Dennis Canavan, Chairman of Yes Scotland's Advisory Board, wrote to Alistair Darling, of the No camp, challenging him to provide evidence for the claims in his campaign's posters that Scotland will receive new powers after a No vote.

Mr Canavan said: 'There is no agreement among the unionist parties on what additional powers, if any, they would devolve to the Scottish Parliament."

Labour, the Conservatives and the LibDems have all committed to giving extra powers to the Scottish Parliament if Scots reject independence.

Meanwhile, Gordon Brown has accused the Tories of being too negative in the debate ahead of a speech in which he will argue it "makes sense" for pensions to be shared with the rest of the UK.

The former PM is expected to say Scottish pensioners benefit to the tune of millions of pound by being part of the Union.

Speaking ahead of tomorrow's event in Glasgow, he said: "I will not be setting out the Tory case for Britain. I will set out a positive forward-looking case for the best future for Scotland, showing how in areas such as pensions it makes good sense to combine having a Scottish parliament with being part of Britain."

The speech will be Mr Brown's first for Better Together, the anti-independence campaign run by his former chancellor, Alistair Darling.