The social care system will "crash" without an immediate injection of cash, former health minister Norman Lamb has warned.
Mr Lamb said soaring care costs combined with NHS trusts facing budget deficits which are "deteriorating rapidly" would have direct impacts on patients.
Speaking at his party's conference in Bournemouth, Mr Lamb warned cancer survival rates would fall and corners would be cut in care for the elderly.
The former care minister told activists that Conservative plans to save £20 billion in the NHS through efficiency savings would fall short.
He also said another £5 billion was needed to fill a shortfall in social care.
Mr Lamb said: "You might be tempted to think that this is a problem we can delay dealing with - provided we stump up enough cash by 2020.
"But the problem is here and now. The impact is already being felt."
He added: "I've been in the department. I have seen the books and I am deeply concerned. If we carry on regardless, the system will crash."
Mr Lamb proposed moving funding of the NHS to its own dedicated tax detailed on every payslip.
His suggestion is not to increase overall taxation initially, but to offset the new levy against existing deductions for income tax and national insurance.
But a dedicated NHS tax could be increased in future and Mr Lamb said this should be controlled at a local level and not by Whitehall.
He said: "Why can't my county of Norfolk decide to spend more on vital services for older people, to improve cancer services or for mental health if it chooses?"
Mr Lamb vowed to consult widely on the NHS tax, which is not official party policy.
He said it would be part of a "national conversation" aimed at taking party politics out of the health care debate.
Mr Lamb said: "Health and care is so important. We must not let party politics get in the way of what is the right thing to do.
"In our debate on Newsnight during the election, both Jeremy Hunt and Andy Burnham agreed. But nothing has happened. If they won't do it, we will."
Mr Lamb repeated his calls for better treatment of mental health, telling the conference it cost the country £70 billion a year in lost productivity, benefits and human misery.
He said: "It's scandalous. And it's stupid. We can do so much better."
In his keynote conference address, Mr Lamb revealed his "internal torture" at deciding to run against Tim Farron for the leadership - a contest in which he finished second by some margin.
But he said: "Travelling across the country to take part in hustings - all 27 of them - and speaking to so many members - and so many new members - was a real honour.
"I'm genuinely humbled to have received the backing of so many of you. Thank you."
Elsewhere on day four of the Liberal Democrat party conference, activists adopted a motion condemning Tory policy on right to buy for encouraging "social cleansing".
Official policy adopted by members demanded 300,000 homes a year and 10 new garden cities in England.
Later, emotional tributes will be paid to the late former leader Charles Kennedy.
Mr Kennedy, who died in June, led the party from 1999 to 2006 and to its greatest electoral success.
A tribute film will be shown and there will be speeches from senior party figures.
Copyright (c) Press Association Ltd. 2015, All Rights Reserved. Picture (c) Ben Birchall PA Wire.