Independence would be a "leap in the dark" for Scots who rely on pensions and other benefits, a leading Labour MP has claimed.

Shadow Scottish secretary Margaret Curran said a Yes vote in September's referendum would lead to "massive cuts in public spending" north of the border, insisting the least well off in society would be hit hardest.

She was speaking as Better Together, the cross-party body campaigning to keep Scotland in the UK, published a new paper on pensions.

But SNP MSP Christine Grahame said independence would allow Scotland to do "even more to provide a fair deal" for pensioners.

The Better Together report said spending on benefits for senior citizens in Scotland is expected to rise by 3.4% of GDP in the next 50 years, "significantly more" than the forecast 2.3% increase for the UK as a whole.

"Benefit spending in Scotland is around 2% higher per head of population than for the rest of the UK," the paper said.

"The UK social security system pays £17.7 billion in benefits to 2.8 million people in Scotland - equivalent to £3,335 for each of us. Over half our population receive UK social security payments and currently rely on these payments to sustain their household finances."

It added: "Overall, Scotland spends £60 per head more on pensions and benefits than the UK as a whole - this is based on individual need, but the overall costs are also reflected in the costs of provision. We spent more on pensions, because we have proportionately more older people, and on things like invalidity benefit because of our industrial history."

But it said if Scotland votes to leave the UK, it would also "leave that UK social security system".

The report continued: "The number of Scottish pensioners will grow from 1 million to 1.3 million over the next 20 years. In Scotland we have a shrinking working-age population. This puts real pressure on the cost of pensions but also reduces the budget available for benefits.

"The Nationalists still have few answers on welfare. But we can be sure that going it alone would mean less to spend on our welfare system, additional costs spent on administration and a population time-bomb ready to hit just as our public finances are projected to decline."

Ms Curran said: "The experts are clear - if we leave the UK we would have to make massive cuts in public spending. This would hit those who have the least the hardest. The SNP Scottish Government's own expert group has warned of serious risks to benefit payments if we leave the UK.

"Leaving the UK would be a leap in the dark for those reliant on benefits or their pension. By saying no thanks to separation, we can make sure that we protect the most vulnerable in our society from the massive cuts that would have to happen in a separate Scotland."

Labour MSP Jackie Baillie said: "By pooling and sharing our resources across an economy of more than 63 million people in the UK, rather than just five million people in Scotland, we are better placed to support the most vulnerable in our society.

"Even though Alex Salmond has been campaigning for separation all his life, he still has no plan for how welfare would be paid for or delivered in a separate Scotland. Independence would put the pensions and benefits of millions of Scots at risk. That's a risk that isn't worth taking."

But Ms Grahame insisted: "The Scottish Parliament has been good for older people in Scotland - allowing us to make greater strides in tackling the scourge of pensioner poverty, and with gains such as free personal care for elderly people making a real difference to people's lives.

"On these key issues for older people, the Scottish Parliament has taken strong and decisive action - and the results speak for themselves. But with the full powers of independence we could do even more.

"After a Yes vote we could take further action to support our senior citizens - making sure that pensions keep pace with the cost of living, holding a review of the pension age to make sure it is right for Scotland, and ensuring a fairer approach to public sector pensions.

"While the UK Government chooses to spend tens of billions on nuclear weapons, while too many pensioners are struggling to make ends meet, independence would allow Scotland to take a different path.

"With the full powers offered by a Yes vote we could build on the gains of devolution and do even more to provide a fair deal for Scotland's older people."

Bob Thomson, a former chairman of the Scottish Labour Party who is backing a Yes vote, said: "It is a sad day when the Labour Party are praising a Tory pensions and welfare system that is being dismantled before our eyes - no wonder over a third of Labour voters already plan to vote Yes in September, because we can deliver a better deal for older people and a genuine welfare state in an independent Scotland."

He added: "It is extraordinary that Labour are describing as secure a UK welfare system which is cutting £6 billion from welfare support in Scotland, directly cutting benefits to women, children, disabled people and poorer pensioners and which will mean that an additional 100,000 children could be in poverty by 2020, according to the Child Poverty Action Group.

"Margaret Curran herself has called the Tory welfare cuts 'brutal' which will cause 'misery for families across Scotland' - yet the Labour leadership prefer Tory government in Scotland to self-government with independence. The outlook under the Westminster system is a generation of austerity, compared to the opportunity to build a new prosperity in Scotland by voting Yes.

"What Margaret Curran and Jackie Baillie are saying is meaningless and wrong. Pensions will be paid on time and in full in an independent Scotland, triple-locked to at least keep pace with the cost of living, and we will also have the powers to review Westminster's unfair plans to increase the age when people get the state pension."

Mr Thomson said: "Scotland has generated more tax per head than the UK as a whole in every one of the last 33 years - to the tune of £800 per head extra in 2012/13.

"The reality is that spending on welfare, including pensions, is more affordable for Scotland than for the UK as a whole - and indeed spending per head on the state pension is lower in Scotland than in the UK. The figures show that 38% cent of Scottish tax revenues are spent on social protection, including pensions, compared to 42% for the UK as a whole."