Boris Johnson has suffered a defeat in the Lords which would force the Government to reveal the scale of the NHS staffing crisis.
Peers backed a cross-party bid to require ministers to publish independent assessments of workforce levels by 171 votes to 119 - majority 52.
More than 100 health and care organisations have backed calls for a new legal requirement for the NHS to commission independent assessments of staff numbers needed to cater for patient demand.
The Government previously ordered Tory MPs to vote against the amendment to the Health and Care Bill.
It comes as NHS England figures today showed more than 110,000 posts lay vacant - the highest figure since before the pandemic began.
The Government has pledged to tackle the enormous treatment backlog, which currently stands at a record 6.1 million people on waiting lists in England.
But there is no official published analysis on how many of which staff will be needed to clear the backlog.
Tory former health minister Baroness Cumberlege, proposing the amendment, said that staffing is "the single greatest problem facing the NHS".
She warned it would be impossible to tackle the vast backlog cause by the pandemic without better workforce planning.
She said: "Staff numbers may be rising but so is the backlog. The current NHS waiting list stands at 6.1 million and it is rising.
"We need workforce capacity that can meet not only today's demand but that of the future also. It takes many years to train nurses, doctors and health professionals.
"We do need a long-term view. Without that we are flying blind."
Relying on agency and bank staff is only a "sticking plaster solution", she warned.
Former NHS England chief Lord Stevens echoed the concern.
He said: "We need this amendment because we need to look beyond the end of our noses, and to vote against this amendment would be to cut off our noses to spite our face."
Ex-Test and Trace boss Baroness Harding said: "Our NHS people are exhausted. They have lost hope that we understand what it is like on the ground for them.
"By passing this amendment we will give them hope."
Health minister Lord Kamall insisted that NHS staff were at the heart of the health service.
He said: "It is right that we ensure that we have the workforce that we need for the future to keep delivering the world-class, safe and effective health and care."
Lord Kamall pointed out the Government had commissioned "a robust, long-term, strategic framework" for the health and social care workforce.
It comes after the NHS issued a plea for "reservist" volunteers to help boost staff numbers.
The programme is aimed at former health professionals and people with no experience of healthcare work.
Thousands of people will be paid for their help and will be expected to work around 30 days of the year.
