Boris Johnson has been criticised for saying "too many care homes didn't really follow the procedures" during the coronavirus outbreak.
The PM was responding to the head of NHS England's call for reform in social care within a year.
Mr Johnson said it was "important to fund" the sector, but it needed to be "properly organised and supported".
Shadow care minister Liz Kendall said "terrified" staff had been "told by the prime minister that it's their fault".
She told the BBC that care workers had been "abandoned" by the government, adding: "This attempt to shift responsibility is a new low."
Care home providers have also condemned the comments by Mr Johnson, calling on him to start "turning the dial up on reform and down on blame".
Care homes have been at the centre of the coronavirus crisis in the UK.
Almost 30,000 more care home residents in England and Wales died during the outbreak than during the same period in 2019 - with two-thirds of those deaths attributed to the virus.
Responding to the criticism, a No 10 spokesman said care homes had "done a brilliant job under very difficult circumstances".
He added: "The PM was pointing out that nobody knew what the correct procedures were because the extent of asymptomatic transmission was not known at the time."
'Learning lessons'
NHS England head Sir Simon Stevens told the BBC's Andrew Marr on Sunday that coronavirus had shone a "very harsh spotlight" on the "resilience" of the care system.
He said the government needed to "decisively answer" how high quality care could be provided long-term, and make that plan within a year.
Asked about the comments, Mr Johnson said: "One of the things the crisis has shown is we need to think about how we organise our social care package better and how we make sure we look after people better who are in social care.
"We discovered too many care homes didn't really follow the procedures in the way that they could have but we're learning lessons the whole time.
"Most important is to fund them properly... but we will also be looking at ways to make sure the care sector long term is properly organised and supported."
Johnson needs the care sector on side
Analysis by Helen Catt, BBC News political correspondent
After 30,000 excess deaths were recorded in care homes in England and Wales in three months - two thirds of them directly attributed to Covid-19 - the government's response to the pandemic in the sector is coming under increasing scrutiny.
The government maintains it threw a "protective ring" around care homes - a claim critics say is at odds with failures in providing personal protective equipment (PPE) and allowing hospital patients to be discharged without being tested.
Boris Johnson has promised to "fix" the social care system, although the government has yet to set out a precise plan for how.
"Solving" social care is a problem that has beaten many a government before this, so if the prime minister is to succeed, he is going to need the sector on side.
Whether his comments are an attempt to shift the blame, or just a case of unfortunate wording, run-ins like this won't help.
Labour's Ms Kendall said the outbreak had been "frightening, exhausting and traumatic" for care staff who had "seen horrific things".
She told the BBC the PM's comments were a "terrible thing to say".
Acting leader of the Liberal Democrats, Sir Ed Davey, also said Mr Johnson "should be ashamed" and "must take responsibility".
Vic Rayner, the executive director of the National Care Forum - which represents 120 social care charities in the UK - welcomed Mr Johnson's acceptance of the need for better funding in the sector.
But she said his comments on procedures "are neither accurate nor welcome", adding: "Care providers have moved to adopt these new procedures consistently, at pace and with integrity."
he Independent Care Group said the vast majority of providers had "done their absolute best in the face of slow and conflicting advice".
The group's chairman, Mike Padgham, added: "We should not be getting into the blame game and it is wrong to criticise care and nursing homes at this time.
"Care providers may not have got everything perfect but neither has the government. For far too much of this pandemic, providers were operating in the dark over what they ought to do and with one arm behind their backs in terms of the support they were given.
"In those circumstances, they have worked miracles."
Ahead of December's election, the Conservatives pledged an extra £1bn per year for social care in England over the next five years.
Mr Johnson has also stated on a number of occasions he would bring forward a plan on social care reform, but it has yet to be published.
The government has given an extra £3.2bn in emergency Covid-19 funds to English councils, which can be put towards helping with social care costs.
Ministers have also promised an additional £600m for care homes to help with controlling infections.
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