By Belinda Goldsmith
LONDON (Reuters) - Former British Prime Minister Gordon Brown is stepping up his efforts to stop Scotland leaving the United Kingdom as opinion polls show growing support for independence.
Brown, a Scot, will on Tuesday make his first speech in support of the cross-party Better Together campaign, warning Scots of a possible deficit in pension funds if they opt out of the 307-year old union in a September 18 referendum.
Polls show growing support for independence with the gap between the two sides narrowing to just a few percentage points after a split seemed unattainable a year ago.
Brown, a former Labour leader who has kept a low profile since losing the 2010 general election, said he will set out the case for why Scotland should stay in the United Kingdom in a speech at Glasgow University.
"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," Brown said ahead of the speech.
Campaigners believe working-class Labour supporters could be key to the referendum with both sides seeking to woo Labour voters as well as women, who opinion polls suggest are more likely to oppose independence.
Brown is expected to give statistics showing that Scottish pensioners could receive 500 million pounds ($840 million) less a year in benefits if the country goes it alone.
He will also warn that Scotland's pension bill could soar as the number of pensioners rises to 1.3 million from one million.
Brown last month said he intended to play a major role ahead of the September vote, but this is his first foray into the intensifying cross-party bid to stop independence.
Brown has acknowledged most of Scotland's 5.3 million population want change although not necessarily separation as advocated by the ruling Scottish National Party (SNP) that trounced Labour at the 2011 Scottish parliament elections.
An ICM poll on Sunday showed support for independence unchanged from a month ago at 39 percent while opposition fell four points to 42 percent. A Survation poll put the pro-independence side at 38 percent, up one point from 10 days ago, and pro-unionists on 46 percent, one point lower.
Better Together, led by former Labour British finance minister Alistair Darling, has been accused of being too negative. This week it will launch a "No thanks" advertising blitz and Labour leader Ed Miliband will lead a visit by his top team to Scotland later in the week.
Scotland set up its own parliament in 1999 after a referendum on devolution, with London handing over powers for issues such as education, health, transport and housing.
Brown argues that the devolved Scottish parliament should be given more powers rather than break up the union of England, Scotland, Wales and Northern Ireland.
But the SNP, led by Alex Salmond, argues independence would let oil-rich Scotland decide its own path rather than having often unpopular policies imposed by lawmakers in London.
Salmond said the latest polls and new advertising campaign showed the No side's campaign was backfiring even after playing all their trump cards.
The anti-independence campaign has focused heavily on warnings that Scotland would lose the pound and have to reapply for European Union membership if it quit the United Kingdom, and the business sector would suffer.
"The No campaign is in a panic because they are seen as negative and unbelievable," Salmond said in a statement.
($1 = 0.5949 British Pounds)
(Reporting by Belinda Goldsmith; Editing by Ruth Pitchford)