LONDON (REUTERS) - The British government will step up its campaign against Scottish independence this week when a senior Treasury official travels to Edinburgh to try to dispel "myths" surrounding claims made by supporters of secession.

Danny Alexander will say the Scottish government's assumptions about oil revenues are over-optimistic and accuse it of failing to acknowledge the impact of Scotland's ageing population on the country's finances, the Treasury said in a statement on Sunday.

Ahead of a Sept 18 independence referendum, the British government's campaign to keep the 307-year-old union with Scotland intact is battling to hold on to a shrinking poll lead over nationalists, who say a split would give Scotland the freedom to create a fairer and more prosperous nation.

The nationalists accuse London and its Scottish supporters of running a scare campaign on the currency, European Union membership and public finances to frighten the 5.3 million Scots into staying in the United Kingdom.

Alexander, Chief Secretary to the Treasury and himself a Scot, accuses Scottish leader Alex Salmond of failing to provide detailed costings on oil and pensions. "This week I will be challenging some of the myths of independence - people need to know the facts," he will say on Wednesday, according to extracts of his speech, released in advance by the British Treasury. "The problems of declining oil revenues and an ageing population cannot simply be wished away," he will add.

A Treasury statement said its officials had spent months analysing the likely impact of Scottish independence and would publish the results in the coming weeks.

It added: "The analysis will set out in more detail than ever before the impact of having to absorb the higher spending and lower tax caused by declining oil revenues, an ageing population, the Scottish government's uncosted policy pledges, and the set-up costs of independence in a much smaller budget." Last week, former British Prime Minister Gordon Brown warned of a pensions "time bomb" in Scotland and put the costs of a new pension and benefits system at 1 billion pounds. Pensioners would be better protected, he said, if Britain stayed together and the costs were spread across the UK.

Polls show support for Scottish independence is growing, with nationalists gaining ground but still behind and large numbers of Scots still undecided.