Asked how their underlying condition had been affected since they were asked to shield, around 81,000 patients (4%) said their condition is 'much worse' - and a further 351,000 (16%) said their condition is 'slightly worse', according to figures compiled by the Office for National Statistics (ONS).
Around 457,000 patients - 20% of the total on shielding lists - are no longer accessing aspects of their care such as tests or scans. A further 226,000 (10%) are no longer accessing any care, the data show.
Meanwhile, despite the vast majority of patients reporting that they followed shielding advice, one in 20 of those advised to shield reported that they had experienced symptoms of COVID-19 - high temperature or new, persistent cough - during the pandemic.
The findings, based on a behavioural survey of 4,149 clinically extremely vulnerable patients, are the latest to show how the pandemic is likely to impact on patients' health and GP workload over the coming months.
The figures come as the government wrote to patients on shielding lists to outline changes to guidance announced last week. Restrictions for patients asked to shield will be reduced from 6 July, and scrapped completely from the start of August.
In an accompanying letter to NHS staff, CMO Professor Chris Whitty and NHS medical director Professor Stephen Powis said: 'We recognise that the significant new pressures the NHS is under and the changing pace of guidance and requests that we make of you in support of your patients, are significant.'
The letter sets out a series of FAQs intended to support NHS staff in conversations with patients - after warnings from the RCGP that practices would face thousands of questions from patients confused by the impact of changes to shielding rules.
GPonline reported in April that patients were waiting more than a month for urgent cancer checks, while NHS leaders urged patients not to put off contacting GPs amid evidence that millions of patients were avoiding seeking care early in the pandemic.
Despite a deterioration in the condition of many patients on shielding lists, the ONS polling also found that 73% of patients on the lists said their condition was about the same, while 7% said it was better or slightly better.
The data also show that the vast majority of patients identified as at high risk had observed shielding advice - with 62% saying they had done so 'completely', 33% 'mostly' and the rest 'partially'.
For more than 1.5m patients shielding from coronavirus (67%), care had continued as normal - while for 2% it had increased.
Wessex LMCs chief executive Dr Nigel Watson told GPonline that it was important for patients who felt their condition had worsened to come forward and seek help from their GP.
He said that for many if their condition had worsened only slightly, it may be relatively straightforward to support these patients to improve. Practices had worked hard to contact patients and to mitigate risk, he said.
The New Forest GP added: 'In the context of what we have been dealing with, for 80% of patients shielding to have no deterioration or an improvement in their condition is good.'
However, he warned that as workload returned to normal or above-normal levels in general practice following the peak of the pandemic - including managing at-risk patients whose condition had deteriorated - hospitals must not be allowed to dump tasks onto primary care that practices do not have the capacity to manage.