Boris Johnson has been accused of over-promising on the country's Covid-19 inoculation programme but ministers insist the "Herculean" target of vaccinating two million people per week is achievable.
The government is aiming to vaccinate 13.9 million people in two months, linking the end of lockdown with the level of immunity in vulnerable groups.
About 1.3 million Britons have been vaccinated across the UK since December 8, including 23 per cent of those over 80 years old in England.
Vaccines Minister Nadhim Zahawi admitted the target was “stretching” but was confident it could be met.
But pharmacies have questioned the government’s ambitions, revealing they had been snubbed for the initial distribution despite having the capacity to deliver 1.3 million shots a week.
Leaked documents also showed Public Health England decided it would not work on Sundays to deliver the vaccines to hospitals.
Labour leader Sir Keir Starmer piled on the pressure during a televised address on Tuesday night, claiming it would be another example of Downing Street “over-promising and under-delivering” if the government failed to meet the target.
He called for a "massive, immediate, and round the clock" vaccination campaign.
"We were the first country in the world to get the vaccine. Let's be the first in the world to get our country vaccinated,” Mr Starmer said.
Simon Dukes, chief executive of the Pharmaceutical Negotiating Services Committee, said pharmacies were “desperate” to distribute the vaccine but their offers of help had fallen on deaf ears.
The bulk of inoculations are currently taking place in hospitals but there are plans to expand the programme, with several new vaccination centres becoming available next week.
Mr Dukes said 11,400 pharmacies across the country were “ready and willing” to help.
"Rather than scrabbling around trying to find retired GPs and nurses and anyone who has possibly dated skills, you've got an army of thousands of pharmacists up and down the country who administer the flu jab every winter," he told The Telegraph.
“We’ve been telling the NHS that we’re ready, willing and desperate to help. But we’ve been met by a de facto silence.”
Mr Zahawi denied the government had snubbed pharmacies and said delivery was gradually being expanded in a safe way. He said the UK would need to vaccinate more than two million people a week to meet the target.
"It is a Herculean effort," he told Sky News. "It is a stretching target no doubt, but I'm confident that with this plan that the NHS have put together that we will deliver this.”
Prof Chris Whitty, England's chief medical officer, said expanding the vaccination programme was "realistic but not easy".
“The NHS is going to have to use multiple channels to get this out but they are very determined to do this, but that does not make it easy,” he said.
The programme is being helped by the change to the dosage regimen – two doses of the Pfizer/BioNTech vaccine can now be delivered within a three-month window rather than three weeks.
This is against the recommendation of the World Health Organisation, which said on Tuesday people should get two doses of the Pfizer/BioNTech vaccine within 21 to 28 days.
The manufacturers themselves also sounded warnings about the dosing schedule, claiming there was “no data” to support its effectiveness.