The spread of coronavirus in the UK could have been slowed with earlier quarantine restrictions on arrivals, a group of MPs has said.
The Home Affairs committee said a lack of border measures earlier in the pandemic was a "serious mistake".
It added ministers had underestimated the threat of importing the virus from Europe as opposed to Asia.
But a Home Office spokeswoman said the committee were "incorrect in their assertions".
She added: "All of our decisions throughout the pandemic have been guided by the science, with appropriate measures introduced at the right time to keep us all safe."
In their report, the committee backed a decision not to close the UK's borders in the early stages of the crisis, given the "large number" of returning British nationals.
But it added that a requirement for people arriving from certain countries to quarantine, introduced in early June, should have come in earlier.
Since then, those arriving in the UK have to self-isolate for 14 days or face the threat of fines, with each of the UK's four nations compiling a list of exempted countries where this does not apply.
During February and early March, all passengers from Hubei Province in China and certain areas of South Korea, as well as Iran and later Italy, were asked to self-isolate for 14 days on arrival.
The MPs criticised a decision not to include Spain on this early list, adding that government advice had initially focused on Asian countries and did not "recognise soon enough" the risk of importing the virus from Europe.
They added that a later decision - on 13 March - to end self-isolation advice for international arrivals not displaying symptoms had been "inexplicable".
The Home Office said this advice was replaced by guidance advising all people in the UK, including arrivals, to self-isolate if they developed symptoms.
Citing evidence from scientific studies, the MPs said it was likely that thousands of infected people then arrived in the UK before full lockdown came in 10 days later.
"It is highly likely that this contributed to the rapid increase in the spread of the virus in mid-March and to the overall scale of the outbreak in the UK," they added.
"The failure properly to consider the possibility of imposing stricter requirements on those arriving - such as mandatory self-isolation, increased screening, targeted testing or enforceable quarantine - was a serious error."
This is the second Parliamentary report in a week that's accused the government of serious errors. Last week's criticised how hospital patients were discharged to care homes without a Covid test.
That and today's report amount to the same accusation - poor or inexplicable decisions that didn't help slow the march of the pandemic.
The MPs cite examples from around the world where countries were requiring passengers arriving in that country to comply with stringent quarantine or monitoring measures.
The government insists that its general message from 13 March to the public to stay at home, if they had symptoms, worked.
But that recommendation was not the same as clear guidance, or an absolute legal requirement, for passengers to self-isolate even if they were feeling perfectly well.
And that, say the MPs, meant travellers in March were able to arrive and move about much more freely at a critical moment in the spread of the virus across the UK.
The committee added that the decision to withdraw self-isolation advice was "very different from countries in similar circumstances".
It concluded that countries that instead introduced tougher border measures, such as Singapore, had been "proved justified in doing so".
It said an official estimate used to justify the UK's approach - stating that only 0.5% of domestic infections had been imported from overseas - was not calculated until late March.
But the MPs point out that the proportion of cases was likely to have been "substantially higher" when blanket quarantine advice was lifted earlier that month.
They backed the mandatory quarantine rules introduced in June, and said ministers should consider greater testing of arrivals at the UK's borders.
Labour MP Yvette Cooper, who chairs the committee and is a former shadow home secretary, said the lack of stronger quarantine rules in March "did make the epidemic worse".
She said the new guidance introduced in mid-March "didn't cover anybody who was asymptomatic, anybody who wasn't sure what the symptoms were".
"At a time when other countries were introducing stronger border measures, the UK was lifting them," she told BBC Radio 4's Today programme.
"We've seen no science behind that decision at all - and it's that lack of science, lack of transparency that's so concerning".
But a Home Office spokeswoman said the government had followed the scientific advice.
"And with passengers numbers significantly reduced, the scientific advice was clear that quarantine measures for those entering the country from abroad would be most effective when the UK has a lower level of infection," she added.
"Therefore, as the virus was brought under control here, border measures were introduced on 8 June to protect public health and help avoid a second peak that would overwhelm the NHS."