The source told Sky News the final stage of lockdown easing will be postponed until July 19.
The Prime Minister is expected to plead the nation for “one last heave”, according to the source.
They added: “It is a straight race between the vaccine and the virus.”
The four week delay is believed to be caused by increasing coronavirus case numbers and fears over the Delta variant, which first emerged in India.
The added delay will allow millions of people to receive life-saving doses of the vaccine.