Reuters/Dundee

Former British prime minister Gordon Brown will set aside a long-running feud on Wednesday to make his first public appearance with Alistair Darling, the leader of the campaign to keep Scotland part of the United Kingdom.

With three weeks to go before a historic independence referendum on Sept. 18, the two Labour party politicians, who fell out while in government from 2007-10, will appear in Dundee to argue that Scotland would be better off staying in the UK.

The joint appearance by two of Scotland's most high-profile politicians comes as postal voting gets underway and as the campaign ratchets up with independence supporters struggling to catch up with their rivals in the polls even though their leader, Alex Salmond, won a final TV debate on Monday.

Darling was Britain's finance minister from 2007 to 2010 under Brown, but his comments on the 2008 financial crisis and subsequent criticism of Brown's leadership soured relations between the two.

Anti-independence "Better Together" campaign director Blair McDougall urged postal voters to focus on the uncertainty that a vote for independence would bring with no clear decision on what currency would be used and warned of the possible impact on pensions and public services.

"For the hundreds of thousands of Scots who will make up their minds and vote over the next few days the big questions are the economic ones. Gordon and Alistair are two of the biggest economic figures in the UK," he said in a statement.

Their joint appearance came as more than 100 business leaders joined forces in the biggest intervention by Britain's business community in the referendum debate so far, signing a letter to oppose Scotland ending its 307-year ties with England.

"As job creators, we have looked carefully at the arguments made by both sides of the debate," the letter said. "Our conclusion is that the business case for independence has not been made."