Boris Johnson has been accused of trying to shift the blame for coronavirus deaths onto care homes.
The prime minister said on Monday that "too many care homes didn't really follow the procedures".
His words sparked fury in the care home sector, with one charity boss calling them "clumsy and cowardly".
Health secretary Matt Hancock said care homes had done "amazing work" during the crisis and rejected Labour calls to apologise for the PM's remark.
"The PM was explaining that because asymptomatic transmission was not known about, the correct procedures were therefore not known," Mr Hancock said in the House of Commons.
He said the government had been been "constantly learning about this virus from the start and improving procedures all the way through".
Appearing in the House of Lords, Communities Minister Lord Greenhalgh admitted that the guidance given to care homes during the early stages of the pandemic was "not as clear as it could have been".
And a No 10 spokesman said the PM would not be apologising for his remarks, and said the government had "put in place rigorous testing and additional funding".
But Mark Adams, who runs the charity Community Integrated Care, told BBC Radio 4's Today programme the prime minister's comments were "cowardly" and a "travesty of leadership".
He added: "If this is genuinely his view, I think we're almost entering a Kafkaesque alternative reality where the government sets the rules, we follow them, they don't like the results, they then deny setting the rules and blame the people that were trying to do their best."
Nearly 20,000 people are confirmed to have died of coronavirus in care homes in England and Wales since the beginning of the outbreak.
The National Care Forum said Mr Johnson's remarks were "frankly hugely insulting" to care workers.
Vic Rayner, executive director of the forum which represents 120 social care charities, told BBC Newsnight that care homes followed the guidance "to the letter" but the government's attention was focused on hospitals.
Labour's shadow health secretary Jonathan Ashworth urged the government to apologise for the prime minister's "crass remarks".
"Care providers were sent conflicting guidance throughout this outbreak, staff could not access testing until mid-April and are still not tested routinely, PPE supplies have been inadequate, thousands of families have lost their loved ones in care homes to this disease, care workers themselves have died on the front line," he said during an urgent question to Mr Hancock in the Commons.
"Can he understand why people are so insulted by the PM's remarks when he said too many care homes didn't really follow the procedures?"
Behind the scenes in the government, there is a frustration the care sector has escaped largely blame free from the crisis.
Care homes are not government-run. On the whole they are owned and operated by private firms.
As you would expert in a network of more than 14,000 homes there is a variation in performances and practices.
Not all care homes have seen outbreaks - and that, of course, means questions should be asked. But the sector is right to complain that guidance, certainly at the start, was changing all the time.
The big national effort on PPE was focused on the NHS, leaving some homes severely lacking in equipment as their supply chains dried up or could not cope.
The roll-out of testing was slow - it is only now that residents and staff are to get regular testing, vital if those who are infected but don't show symptoms are to be spotted.
This virus is very tricky to contain and the UK is not alone in struggling to protect care homes.
But no debate would be complete without mention of funding.
The overhaul of the system has been talked about for years, but nothing has been done, leaving some services in a precarious position. The virus has certainly exploited that.
It comes as the Care England, the largest body representing independent care homes, accused the government of dragging its feet over issuing new guidance for visitors to care homes.
Chief executive Martin Green said: "We are at a loss to know why the Department of Health and Social Care (DHSC) is incapable of making swift decisions at a time of crisis.
"As the country unlocks, care providers are in the dark as to what is permissible in terms of visitors to their residents, or indeed residents leaving their homes on visits.
"This should have been a priority for the DHSC given that care homes are central to fighting this dreadful pandemic".
Imelda Redmond - the national director of Healthwatch England which champions health and social care users - told BBC Radio 4 that "the issues that underlie all of this have been there for a long time".
"There has been underinvestment in social care for many years - and there needs to be quite significant amounts of reform - all those fault lines have been laid bare in this pandemic.
"We need to get a grip to this before we enter winter and perhaps a second wave."
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.
Asked on Monday about Sir Simon's 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."
Ahead of December's election, the Conservatives pledged an extra £1bn per year for social care in England over the next five years.
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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