Labour and the Conservatives have rejected a plea from doctors’ leaders to “put their money where their mouth is” and guarantee the NHS an extra £8bn a year by 2020.

The two main parties have spurned the pre-election demand by an alliance of bodies representing GPs, hospital consultants, A&E specialists and other doctors that they commit to finding the “substantial” sum if they form the next government. The Lib Dems have pledged the extra funding.

Simon Stevens, NHS England’s chief executive, has made clear that the service will need at least an extra £8bn a year by the end of the next parliament if it is to remain viable in the face of mounting pressures and unprecedented demand for care linked to the ageing population. That would take the NHS’s budget, currently £113bn, to over £120bn.

In a letter to the Guardian, the leaders of six medical royal colleges warn that without at least the extra £8bn, the NHS’s future will be at risk. They have increased the pressure on the parties over what is expected to be a key issue in the forthcoming election campaign by stressing that their recent pledges of extra cash risk being seen as mere “window dressing”.

The letter says: “All the major parties say they want to maintain the NHS as a service that delivers excellent patient care, free at the point of access. But voters, concerned about the future of their health service, will want to see the parties vying for their support put their money where their mouth is and pledge to increase investment in the NHS in line with the sums for which the politically-neutral Mr Stevens is calling”.

Signatories include Dr Maureen Baker, chair of the Royal College of GPs, Prof Jane Dacre, president of the Royal College of Physicians (hospital doctors) and Dr David Richmond, president of the Royal College of Obstetricians and Gynaecologists. It has also been signed by the leaders of the equivalent organisations representing anaesthetists, psychiatrists and paediatricians and child health experts in the NHS, and A&E doctors.

They point out that the highly respected Commonwealth Fund, an American thinktank, has rated the NHS as one of the most equitable and cost-effective health systems in the world, “but this status will be put at risk if we don’t fund it adequately.”

The letter adds that: “We therefore call on all the political parties taking part in the general election to commit formally to investing an additional £8bn per year in the delivery of healthcare by the end of the next parliament, in order to safeguard the future of the NHS. Without such a firm commitment, voters may view the promises of the political parties as being nothing more than window dressing.”

Stevens identified the £8bn as the minimum extra amount the NHS would need by 2020 to help close an anticipated £30bn gap in its finances when he launched the NHS Five Year Forward View, a blueprint for its future, last October. Greater NHS efficiency and productivity, as well as adopting new ways of working, will realise the other £22bn, he said.

Norman Lamb, the Liberal Democrat health and care services minister, told the Guardian his party would deliver the £8bn, citing a commitment made in January by party leader Nick Clegg.

“We have already confirmed – and no one else has, to my knowledge – that we commit to raising the £8bn,” Lamb said. “And we also call for all parties to come together this year and support a non-partisan commission to review the budgets of both the NHS and social care. We need to engage the public in a debate about how we sustain the NHS. Remember the £8bn figure depends on some pretty big assumptions about efficiency savings over the next parliament – 2% a year rising to 3%. That has never been achieved before”.

At a Guardian debate last week about the NHS, Lamb described Stevens’ promise of £22bn NHS efficiency gains as an “heroic assumption”, meaning that the health service could end up needing more than the extra £8bn. Senior NHS figures share that belief.

But Labour and the Tories both refused to match the Liberal Democrats’ pledge.

A Conservative spokesman said: “We have committed to protect the NHS budget and increase it in real terms in every year of the next Parliament – and are investing an additional £2bn in the frontline from April, which Simon Stevens called ‘the funding we need for next year’. But you can only have a strong NHS with a strong economy and so Labour would put funding for the health service at risk.”

Party sources highlighted that, despite the precarious financial situation they inherited in 2010, they have since increased the NHS in England’s budget by £5bn in real terms.

Despite having made the NHS their key issue since last summer, Labour also declined to commit to the £8bn.

“Labour is the only party with a fully-costed and funded plan. We will invest an extra £2.5bn a year, on top of the Tory spending plans we inherit,” said a party spokesman.

“The Tories cannot be trusted with the NHS, whatever they say before the election. They broke their NHS promises in this parliament. And their extreme plans for £70bn of spending cuts – which go way beyond balancing the books – would put our NHS at risk.”

Chris Ham, chief executive of the King’s Fund thinktank, said: “An additional £8bn a year by 2020 is the minimum requirement for the NHS is to continue to meet patient needs and maintain standards of care. The commitments made so far by the parties are welcome but in the main do not go far enough. They and the public should be under no illusions – if this funding is not found, patients will bear the cost as staff numbers are cut, waiting times rise and quality of care deteriorates.”