The document could take the form of a vaccine certificate, providing evidence of Covid protection, and the NHS mobile app could be modified to carry the information.
But the prime minister was keen to emphasize the difficulties involved, and it’s an idea that he and other ministers have dismissed in the past.
Boris Johnson said he was confident that all restrictions in England could be lifted by 21st June.
In Scotland, the first minister Nicola Sturgeon has unveiled what she called a “deliberately cautious” strategy with a phased re-opening of the economy.
Huw Edwards presents BBC News at Ten reporting by deputy political editor Vicki Young and home editor Mark Easton.