The government was not "fully prepared" for the "wide-ranging impacts" of the coronavirus pandemic, according to a new report, which found that Brexit both helped and hindered preparations for future crises.
There was a lack of detailed plans for shielding, job support schemes and school disruption, the National Audit Office found.
According to the watchdog, lessons from previous "simulation exercises" that would have helped when COVID-19 hit were "not fully implemented".
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1:19 May: PM establishes COVID inquiry
May: PM establishes COVID inquiry
"This pandemic has exposed the UK's vulnerability to whole-system emergencies, where the emergency is so broad that it engages all levels of government and society," said Gareth Davies, head of the NAO.
"Although the government had plans for a flu pandemic, it was not prepared for a pandemic like COVID-19 and did not learn important lessons from the simulation exercises it carried out."
A government spokesperson defended its handling of COVID, stressing the "unprecedented" nature of the pandemic.
"We have always said there are lessons to be learnt from the pandemic and have committed to a full public inquiry in spring," they said.
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"We prepare for a range of scenarios and while there were extensive arrangements in place, this is an unprecedented pandemic that has challenged health systems around the world.
"Thanks to our collective national effort and our preparations for flu, we have saved lives, vaccinated tens of millions of people and prevented the NHS from being overwhelmed."
The NAO said that resources dedicated to preparing for Britain's exit from the European Union had both a positive and negative impact on planning for future crises.
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2:46 March 2020: Life under lockdown
March 2020: Life under lockdown
In some instances, work on Brexit enhanced the "crisis capabilities" of certain government departments.
But on the flipside, the strain on resources meant the government had to pause or postpone some planning work for a potential flu pandemic.
"Some work areas of the Pandemic Flu Readiness Board and the Pandemic Influenza Preparedness Programme Board, including scheduling a pandemic influenza exercise in 2019-20, were paused or postponed to free up resources for EU exit work," the report said.
Of the Cabinet Office's 94 full-time equivalent staff in its emergency planning unit, 56 were allocated to prepare for potential disruption caused by a no-deal Brexit.
The NAO said this had the result of "limiting its ability" to plan for other crises.
"This raises a challenge for the government as to whether it has the capacity to deal with multiple emergencies or shocks," its report said.
According to the NAO, the pandemic has "exposed a vulnerability to whole-system emergencies".
And while ministers had plans to try and handle a pandemic, many of them were "not adequate" for the challenges that COVID posed, it said.
The report cited the findings of Exercise Winter Willow, a large-scale pandemic simulation exercise from 2007.
This found that business continuity plans needed to be "better coordinated" between organisations, but the NAO said this was "not evidence in most of the plans it reviewed.
In the wake of another pandemic simulation, Exercise Cygnus in 2016, the government noted that "consideration should be given to the ability of staff to work from home, particularly when staff needed access to secure computer systems".
But the NAO said that when coronavirus reached the UK, "many departmental business continuity plans did not include arrangements for extensive home working".
The watchdog found that the government prioritised preparations for "two specific viral risks" – an influenza pandemic and an emergency high-consequence infectious disease.
As a result, there was no plan specific to a disease with the characteristics of COVID, with it being found that scientists considered such a disease "less likely" to occur.
While the report noted that the government was able to use some of the mitigations it had in place when the pandemic struck, such as the stockpile of personal protective equipment (PPE), it was "not fully prepared" for the "wide-ranging impacts" the virus would have on society, the economy, and key public services.
The NAO queried why this was the case, given that the government's 2019 National Security Risk Assessment said that a flu-like pandemic could have "extensive non-health impacts, including on communications, education, energy supplies, finance, food supplies and transport services".
In addition, the watchdog said that there was no agreement in government of "what level of risk it was willing to accept for an event like COVID-19".
COVID crisis – learning the lessons
According to the NAO, it was told by the Cabinet Office that the government's "risk appetite changed" and it "lowered the threshold for the health and societal impacts of the pandemic that it deemed acceptable" as COVID hit.
In conclusion, the NAO found that the pandemic had flagged up the need to beef up the government's risk management process and "national resilience" to be ready for future crises of a similar nature.
It made a number of recommendations to the Cabinet Office on risk management and preparedness.
The watchdog did note that the government had already begun considering how to address the issues raised in its report, for instance through its National Resilience Strategy.
Lobby Akinnola, a spokesperson for the COVID-19 Bereaved Families for Justice campaign group, said the report "confirms what the whole country has long known, that the government was unprepared for the pandemic and that our loved ones might be here today if they had been".
"What's most important now is that the government is able to learn lessons from the inquiry to ensure that we are never in this position again."
Labour's shadow cabinet office minister Fleur Anderson said the report shows that ministers "failed to prepare and they failed the public".
"It is vital that preparedness and planning is addressed in the public inquiry into the Conservatives' mishandling of the COVID-19 pandemic," she said.
Sir Ed Davey, leader of the Liberal Democrats, said: "This is a damning report that reveals a comprehensive failure from the government to prepare adequately for the pandemic."