The Government struggled to contact hundreds of thousands of clinically vulnerable people who were told to shield at the start of the coronavirus pandemic.
The official spending watchdog found officials had to scramble to develop from scratch a system to identify those who might need support while they were stuck at home.
It meant they had had to rely on hospital records that were weeks out of date or with missing or inaccurate telephone numbers.
As a result, some of those shielding had to wait weeks before they were able to get deliveries of food or medicines.
Disability charities blasted the Government over the failure to protect the most vulnerable from the pandemic.
Two thirds of all those who died from coronavirus were disabled, they said.
Louise Rubin of Scope said: “Thousands of disabled people faced months without the support they should have got, because despite being most at risk, they’d been forgotten from contingency plans.
“Despite this, the Government still does not appear to understand the impact of coronavirus on disabled people.
“Many disabled people have been shielding for a year now and are still struggling to get basics such as food and social care.
“The Government must put this right and deliver an emergency support package to protect disabled people’s health and finances.”
The NAO report found a contact centre set up to reach those who had not registered for the scheme online or through a phone line was unable to reach 375,000 people.
Meanwhile, 126,000 people were contacted in error and wrongly advised to shield when they did not need to do so.
Ministers had urged clinically extremely vulnerable people to stay at home for 12 weeks and not go out for shopping, travel or leisure.
The Government offered food, medicines and basic care to those in this category.
By April 12, 1.3m people were identified as eligible for support, with another 900,000 added by May 7.
Gareth Davies, the head of the NAO, said: “Challenges pulling together data meant it took time to quickly identify those needing to shield, and therefore eligible for support.
“Government has learnt lessons from this programme and is better placed should this type of support be required again.”
The NAO said that a 2016 exercise to test for preparedness for a flu pandemic codenamed Exercise Cygnus had not covered the need for vulnerable people to shield.
It meant there was no system in place to allow a fast “sweep” across all patient data to identify those who should be on the list and who may need support.
In all, more than 510,000 people were supported through the scheme to the start of August at a cost to the taxpayer of £308 million – of which £200 million was spent on food boxes.
The Whitehall spending watchdog said that the Department of Health was unable to say whether shielding led to fewer deaths or cases of serious illness among the clinically vulnerable – although it was “likely” that it helped.
Meg Hillier, the chair of the Commons Public Accounts Committee, said: “Government quickly identified the need to support vulnerable people at the onset of the pandemic. But with nothing prepared, Government had to build its shielding programme from scratch.
“Nearly a million people were not eligible for support for weeks while the Government scrambled to gather basic information. Meanwhile, too many vulnerable people were left confused by Government’s mixed messages.