The Better Together chief is due to argue the social case for Scotland rejecting independence when he speaks to the Scottish Council for Voluntary Organisations (SCVO) in Glasgow.
Access to UK National Lottery funding is one example of how funds generated across the UK are used to benefit the third sector, Mr Darling said.
He is expected to tell his audience: “The UK is more than an economic and political union. It is also a social union, enabling us to work together across the four nations of the UK for the benefit of all.
“It is this social union which allows us to tackle injustice and inequality wherever it exists in the UK.
“Pooling and sharing our resources across the whole of the UK enables us to tackle this injustice and inequality far more than we ever could on our own.
“Many of our third sector organisations work across the whole of the UK, from Cancer Research to the British Heart Foundation.”
He will add: “Organisations here today will do extraordinary work overseas in international development programmes. The UK has one of the best and most effective international aid budgets in the world, reducing poverty and eradicating inequality in some of the most deprived parts of our world. Why walk away from that?
“Projects and groups in Scotland have received substantial funding from the National Lottery. It’s a cross border relationship that works, but only because we are part of the UK. Leaving the UK would fundamentally change that.”
But a spokesman for Finance Secretary John Swinney said Mr Darling’s comments were “another bogus attack”.
“The position on the Lottery was detailed in the White Paper, which Mr Darling still seems not to have read - the current licence to run it is in place to 2023, and the National Lottery will continue to operate in Scotland after independence.
“We will ensure that Scotland receives our fair share of funding from the National Lottery.
“At present, the Big Lottery Fund, sportscotland, Creative Scotland and the Heritage Lottery Fund distribute good cause money across Scotland. However, many decisions are still made at UK-wide level.
“In an independent Scotland, all decisions about the distribution of good cause money will be made in Scotland to ensure that the needs of local communities are met.”