A report from the House of Commons public accounts committee said the government 'took too long to identify all clinically extremely vulnerable people'.
It highlighted the 'purely clinical approach' taken initially to compiling the shielding list - omitting key factors such as ethnicity, BMI and postcode - and pointed to the huge expansion of the shielding list in February this year based on a risk tool drawn up by UK academics.
The report also said GPs were inconsistent in judgments over who should be advised to shield - pointing out large variation between areas. In some parts of England, numbers of patients initially identified as vulnerable by NHS Digital increased just 15% after review by local GPs - whereas in other areas the figure increased by more than 300%.
Thousands of patients identified as needing to shield may have 'slipped through the net' and missed out on support, the report added.
People initially identified for support were sent letters, emails and then called by a contact centre that cost £18.4m to set up. The contact centre was unable to contact 800,000 of those identified as needing support and passed their details on to local authorities after a month - but the government does not know whether these people were subsequently contacted successfully.
The BMA said shielding advice had been 'disjointed and unclear' and added hugely to GP workload at a time when the profession was already overwhelmed. The government, however, hit back at the report - calling its findings 'disappointing and misjudged'.
House of Commons public accounts committee chair Meg Hillier said: 'The shielding response in the COVID-19 pandemic has particularly exposed the high human cost of the lack of planning for shielding in pandemic planning scenarios. It also highlights the perennial issue of poor data and joined up policy systems.
'People were instructed to isolate, to protect themselves and others - but the cost of this protection was reduced access to living essentials like food, and an untold toll on the mental health and wellbeing of the already most vulnerable. There are questions still to be answered about the balance between central decision making and local knowledge - the increase in numbers of those advised to shield demonstrate the challenges of trying to deliver this programme centrally, as well as with the data held by the NHS.'
BMA chair Dr Chaand Nagpaul said: 'Throughout this pandemic, it has been some of the most vulnerable people who have been the most disproportionately impacted and the disjointed and unclear shielding guidance has been a factor in this.
'Had we been better prepared ahead of the pandemic, the shielding lists could have been created more quickly and effectively rather than shifting criteria and suddenly increasing the number of people on the list. At a time when GPs were already overwhelmed, this led to a huge rise in workload having to contact large numbers of people within a short time frame without comprehensive guidance.'
Dr Nagpaul criticised 'unclear communication from the government and repeated changes to guidance' - warning this had put public health at risk and added to patients' distress.
The BMA chair added: 'It is important that any future planning should be informed by better engagement with disabled and vulnerable people to understand how their needs can be considered and how best to accommodate this.
'As much as possible must be done to mitigate the negative impact of shielding on doctors and healthcare workers who have faced enormous career disruption and there is a clear and consistent protocols in place to ensure that those shielding are safely reintegrated back to the workplace.'
A government spokesperson said: 'These report findings are disappointing and misjudged. During this globally unprecedented emergency, we worked across multiple government departments to build and deliver an urgent national scheme in record time, identifying 1.8 million clinically extremely vulnerable people and providing them with vital food and medicine to help them shield effectively.
“We made significant efforts to contact people by letter, text and telephone and worked closely with councils to ensure we reached them. Many people chose not to take up the offer of government support as they felt they didn’t need it.
“The initial shielding guidance was agreed by the four UK Chief Medical Officers on the basis of the latest available evidence. Since then we have learned more about the virus and adapted our approach, which has enabled us to protect those most vulnerable by providing them with shielding guidance and prioritising them for vaccination.”