Iain Duncan Smith says an independent Scotland will have to raise taxes or increase borrowing to plug a pensions black hole.

In a speech today, the Conservative Welfare and Pensions Secretary will claim a stand-alone state faces problems with an ageing population and a smaller tax base.

The Treasury has calculated that by 2060 Scotland will have just 1.9 working people for every pensioner, compared with 2.2 in the rest of the UK.

Mr Duncan Smith will say: "The SNP has some serious questions to answer. How would they pay for this? More borrowing? Higher taxes?

"United, the UK is in a stronger position to respond to these challenges, sharing the resources and risks. It gives people certainty there is a secure welfare safety net in place - one which will endure in the future."

He will also highlight a leaked memo in which Scottish ministers privately voiced concerns over the welfare issue.

The UK Government says each working-age person pays taxes worth around £2800 a year to help the state fund pensions and other pensioner benefits, such as the winter fuel allowance. Over the next 50 years, that is estimated to rise by £700 a year. In Scotland, that figure would be £1100 more.

Meanwhile, Citizens Advice Scotland has warned the UK Government's welfare reforms will have a "devastating" impact on benefits claimants. Disabled people and their families stand to lose a total of more than £1 billion in benefit payments, the charity said.