The makers of the vaccine, Pfizer and BioNTech, have warned that they have no evidence their vaccine would continue to be protective if the second dose is given more than three weeks after the first.
Leaving a 12-week gap is allowing Britains vaccine programme to proceed quickly.
Government data published on Saturday showed 5.86 million people have now received a first dose of the vaccine, after a record 478,248 people had the jab in the past 24 hours.
Mr Whitty said on Friday that the longer wait between doses was a public health decision aimed at vaccinating many more people and based on the fact that the great majority of protection comes from the first jab.
The Department of Health and Social Care said the decision on the 12-week gap was made after a thorough review of the data and was in line with the recommendations of the UKs four chief medical officers.
Britain is using the Pfizer vaccine and another from AstraZeneca. AstraZeneca has supported the gap between its jabs, saying data showed an 8- to 12-week gap was a sweet spot for efficacy.
Following Johnsons warnings about the more deadly nature of the new variant, some scientists said on Saturday it was too soon to be clear about what the evidence was showing.
The question about whether its more dangerous in terms of mortality I think is still open, Graham Medley, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, told the BBC.
Reuters