Claims the government put a protective shield around care homes at the start of the pandemic were "complete nonsense," Dominic Cummings has said.
The former government adviser told MPs he was shocked, in April 2020, to find residents were not being tested before going back to care homes.
He said Health Secretary Matt Hancock had previously agreed Covid testing would take place.
The government said it had taken a number of steps to protect care homes.
The prime minister's spokesman said: "We have always been guided by the latest advice at that time, and we have taken a number of steps to protect care home residents and those being discharged from hospitals into care homes."
The PM's former chief adviser, Dominic Cummings, faced questions about early decisions around care homes, at a hearing of the Commons Health and Social Care and Science and Technology committees.
He told MPs: "We were told categorically in March that people would be tested before they went back to care homes. We only subsequently found out that that hadn't happened.
"The government rhetoric was we put a shield around care homes - it was complete nonsense.
"Quite the opposite of putting a shield around them - we sent people with Covid back to the care homes."
Mr Cummings said the situation was made worse because care home staff did not always have adequate PPE or testing equipment.
Nadra Ahmed, of the National Care Association, said Dominic Cummings' remarks about care homes were disappointing.
She told the PA news agency: "It is with great sadness that listening to Mr Cummings, it emerges that our initial thoughts and the evidence that was around us was right - that there was no shield around care homes, there was no thought on the impact on the vulnerable people that we care for."
Ask almost anyone who works in care, those who need services and their families, whether the government put a "protective ring around the care sector from the very start" of the pandemic, as it claims, and their response is one of disbelief - even of derision.
So for many today's comments from Dominic Cummings will have confirmed their memories.
Given the vulnerability of the people they look after, in early March 2020 many care providers started asking for guidance on how to protect against Covid.
With each week that passed those voices got louder. They struggled to get protective equipment, felt official guidance was too slow and had almost no testing.
Some homes complained of being pressurised to take patients from hospitals. And the government only published its plan for social care when deaths among care home residents were already at a peak.
For the families of care home residents who died and care workers, today's testimony will have further underlined the importance of the coming inquiry into what happened.
Shadow social care minister, Liz Kendall, said the government was too slow to act to protect residents and staff.
She added: "Over 30,000 care home residents have died of coronavirus during this pandemic - 25,000 elderly people were discharged from hospitals to care homes without any tests whatsoever, and frontline care workers were left without PPE.
"As we emerge from this pandemic, ministers must put in place a plan to transform social care, and ensure that care homes never again face a crisis of this scale."
Natasha Curry, of the health think-tank the Nuffield Trust, said serious questions needed to be asked about the timing and effectiveness of measures in place for social care.
She said: "The government was reportedly warned as early as January last year that the care home sector in particular would be hit hard by what was known about the virus, yet routine testing, adequate PPE and a plan for the sector came much later.
"Care homes were left often ill-prepared and sadly the extension of testing to social care staff and care home residents in late April came far too late to protect many of the most vulnerable."
A National Audit Office report in June 2020 said about 25,000 hospital patients were discharged into care homes in England at the height of the pandemic, without them all being tested for Covid-19.
After initially saying a negative result was not required before discharging patients from hospitals to care homes, the government later said on 15 April that all patients would be tested.
Regular testing for staff and residents in care homes in England was rolled out in July.