Hertfordshire's public health chief Jim McManus has warned against the easing of Covid-19 restrictions on June 21.
It comes as speculation mounts that Prime Minister Boris Johnson looks set to delay the country's opening up following another sharp rise in cases of the Delta variant, first identified in India.
Ministers are considering putting back the relaxing of controls for up to four weeks – potentially to July 19 – as they race to roll out the vaccine to the remainder of the population.
A final decision is expected to be taken tomorrow (Sunday 13th June) ahead of a formal announcement by the Prime Minister at a news conference the following day.
Though, Mr Johnson who is currently attending the G7 summit in Cornwall, seemed to suggest that the delay would happen.
Mr McManus - who is also vice president of the Association of Directors of Public Health (ADPH) - warns that lifting all measures could lead to an increase in the number of cases and an increase in the number of people needing hospital admission.
Writing on the ADPH website he said it would risk the introduction of new variants to the UK, which could 'undermine' the vaccination programme and 'derail' the path back to normality.
In making his case, Mr McManus says the determination and patience shown by communities during the pandemic has been "remarkable" and he acknowledges that the impact on daily lives "cannot be understated".
But he says this is now a "crucial point" - and that it is vital to be guided by data and not dates.
Doctors and leaders in the British Medical Association (BMA) have also called for the final lifting to be put on hold so that millions more can gain the protection of the vaccine.
BMA council chairman Dr Chaand Nagpaul said: “With only 54.2% of the adult population currently fully vaccinated and many younger people not yet eligible, there is a huge risk that prematurely relaxing all restrictions will undo the excellent work of the vaccine programme and lead to a surge of infections.
“It’s not just about the number of hospitalisations, but also the risk to the health of large numbers of younger people, who can suffer long-term symptoms affecting their lives and ability to work”.
Scientists now estimate that 96% of all new cases of coronavirus are attributed to the Delta variant.
The latest figures from Public Health England (PHE) show there have been 42,323 cases of the Delta variant confirmed in the UK, up by 29,892 from the previous week.
It estimates the strain is 60% more transmissible compared with the previously dominant Alpha (or Kent) variant, and that cases are doubling every four-and-a-half days in some parts of England.
The setback comes as leading figures behind the successful vaccination programme were recognised in the Queen’s Birthday Honours List.
They include the Oxford/AstraZeneca vaccine mastermind Professor Sarah Gilbert and the ex-chairwoman of the UK vaccine taskforce Kate Bingham who are both recognised with damehoods.