It comes after the PM appealed for a million vaccinations per day.
Boris Johnson set a lofty goal for an NHS that presently only manages roughly 500,000 daily vaccination doses.
However, there are concerns that other NHS services would be cut to fund the expanded implementation, while lateral flow tests appear to be in short supply.
On Monday, people lined up around the block at various immunisation sites, while the government advised personnel to work from home ‘if possible'.
Everyone over 18 in England will get booster shots starting this week, the PM revealed on Sunday night.
For the next four weeks, GPs will only focus on urgent requirements and vaccinations, according to Health Secretary Sajid Javid.
As a result of the website's excessive wait times, the NHS in England recommended customers to try again later or tomorrow.
‘The Covid vaccination booking service is currently experiencing unusually high demand and is implementing a queueing system.'
‘Users aged 18-29 should note that booking begins on December 15th.
‘We recommend trying again later today or tomorrow.'
The head of the body representing NHS trusts has claimed the strain on services is ‘not sustainable' as the UK battles Omicron Covid-19 infections.
Elective surgery backlogs, the extension of the booster campaign, and social care demand mean the NHS is ‘busier than ever before,' according to NHS Providers CEO Chris Hopson.
The fact that it is before the traditional winter peak in January, and before any cases really coming into hospitals, which we are now starting to do in terms of Omicron cases, means we are already at beyond full stretch before either of those things occur, in our opinion.
The Prime Minister's appeal for ‘extraordinary effort' comes while personnel are ‘very, very exhausted'.
‘I think people are anxious that this amount of pressure will become normalised and not sustainable,' said Mr Hopson.