An extra penny on income tax is needed for Britain’s “woefully underfunded” health and social care services, according to the former head of the NHS.

Sir David Nicholson, who led NHS England for almost a decade, is among a group of senior health professionals and experts to back an increase in personal tax to provide the struggling services with an extra £6bn a year. The group, which includes past heads of the Royal College of General Practitioners (RCGP) and the Royal College of Psychiatrists, also backs the idea of introducing a “dedicated, transparent NHS and care tax”.

This represents a major boost for the Liberal Democrats, who will back both policies in their election manifesto.

In a letter to the Observer, published online, the group writes: “The NHS and social care are in a state of crisis. Hardworking, dedicated staff, who are fighting to provide high quality, compassionate care, are being undermined because these essential services are being woefully underfunded.

Facebook Twitter Pinterest Sir David Nicholson, the former chief executive of NHS England, is one of the letter’s 26 signatories. Photograph: David Levene/The Guardian

“As people who have worked for many years in the NHS, social care and related fields, we know the impact this is having on patients and on staff morale, on a daily basis.. For these reasons, we strongly welcome the Liberal Democrats’ commitment to raise income tax by 1p, to generate additional, ringfenced revenue for NHS and social care.”

The 26 signatories include Dr Clare Gerada, former chair of the RCGP, Peter Carter, former chief executive of the Royal College of Nursing, Professor Dinesh Bughra, former president of the Royal College of Psychiatrists, and Nick Bosanquet, professor of health policy at Imperial College, London.

The letter adds: “We also welcome [the Lib Dem] manifesto commitments to develop a long-term funding settlement for health and social care. Including bringing together funding into a dedicated, transparent NHS and care tax, establishing an independent body to advise on NHS and care budgets, and convening a cross-party convention on NHS and social care to work with patients, the public and staff, to deliver a sustainable funding settlement for NHS and care services.

“We hope that ahead of 8 June, all parties will set out similarly bold commitments to give the NHS and social care the funding it desperately needs.”

Norman Lamb, the former Lib Dem care minister, said: “The Tories keep saying that they have given the NHS all this money and more money than ever before, but that misleads people. It says nothing about whether it is enough to meet the extraordinary rise in demand and how far we have fallen behind other comparable countries.

“There is a real sense of urgency because the system is close to tipping point. The Care Quality Commission has already issued that phrase for the social care system, but the NHS is in a very fragile state. It has had an extraordinarily tough winter without a flu epidemic and without cold conditions.

“Bluntly, we can’t carry on like this. Only the Lib Dems have a clear and credible plan for how we raise the money and get it into the NHS and the care system.”

The idea of a hypothecated NHS tax has been gaining ground, with former Treasury permanent secretary Nick Macpherson recently raising the idea.

A draft of Labour’s manifesto, leaked last week, showed it plans a major cash injection for the service, by increasing tax on earnings of more than £80,000 a year and on private medical insurance.

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Theresa May is being urged by senior Tories to state that she will increase taxes after the election, amid rising pressures on public spending and large funding cuts in Whitehall departments. Some senior party figures want her to signal in the manifesto that she is prepared to use tax rises to take on some of the major challenges facing Britain, including pressure on the NHS and social care.

“It is time for us to be honest and Theresa May can be honest – with no real opposition – in saying we have some hard choices ahead of us,” said one former senior minister. “Tory candidates support not making mad commitments on tax, but it has to go further than that.

“We are beginning to see what happens after seven years of trying to balance the books. The police are finding it harder, there are issues around fair funding for schools, and social care is creaking at the seams. This is about starting a national conversation.”