Prime Minister Boris Johnson said he would not rule out introducing mandatory Covid passports for people attending sporting events in England this winter.
In a news conference outlining what would be done if Covid-19 cases continue to rise, he said Covid passports were only part of "Plan B".
He said "they might be appropriate" for venues with "closely packed crowds".
"We will keep further measures in reserve," Johnson said.
Covid passports mean people would be required to show proof - whether of double vaccination, a negative Covid test or finishing self-isolating after a positive PCR test - in order to gain entry to crowded events.
If introduced, Covid passports would be required for outdoor sports stadiums with an attendance of 10,000 people or more.
The PM added: "Some events have been using [mandatory checks] over the summer, very effectively, to get going and I think it would be sensible for a government not to rule that out."
In the Premier League, Brighton, Chelsea, and Tottenham have already introduced mandatory checks for supporters attending games at their stadiums. Manchester United have also introduced Covid-19 spot-checks on matchdays at Old Trafford.Covid winter plan to tackle coronavirus in England unveiled
Speaking on Tuesday, Johnson said "Plan A" for dealing with the Covid pandemic will focus on completing the vaccine rollout as well as continuing to increase testing.
However, Johnson said that if pressure increases on the NHS then introducing mandatory face coverings, Covid passports and telling more people to work from home is "an option" as part of "Plan B".
"We do not see the need now to proceed, for instance, with mandatory certification but will continue to work with the many businesses that are getting ready such a scheme," he said.
"Indeed, over 200 events have already used Covid certification voluntarily. It's just not sensible to rule out completely this kind of option now. When we must face the fact that it might still make the difference between keeping businesses open at full capacity or not."
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