Fans attending sports venues with capacities of about 20,000 or more may be required to show proof of full vaccination from the end of September, under plans being considered by the government.
Prime Minister Boris Johnson announced on Monday there was a plan to mandate certification at nightclubs and "other venues where large crowds gather".
The BBC understands the definition of what constitutes a large crowd and what type of venues would fall under this label is yet to be determined.
Downing Street said on Tuesday that its focus is on "closed indoor settings".
The prime minister's official spokesperson said: "We're not seeking to draw lines around particular settings.
"I think Professor [Jonathan] van Tam and the PM talked about the sort of areas where we'd be looking at enclosed spaces where large groups of people are brought together.
"We're going to be looking at the latest data which currently is very focused on nightclubs. But we'll be looking at the data both here and globally to consider what's the best approach."
A government spokesperson said: "As the Prime Minister said, we reserve the right to do what is necessary to protect the public and reduce transmission, including mandating Covid certification in certain settings.
"We are working closely with organisations that operate large, crowded settings, where people are likely to be in close proximity to others outside their household, to encourage the use of the NHS Covid pass, where appropriate."
Indoor venues are expected to be treated differently to outdoor ones, and sports organisations are known to be seeking clarity on the matter.
The Racecourse Association (RCA), for example, said it would be speaking to the government and industry stakeholders "to understand the detail behind this plan", and said it would be a "significant logistical operation".
The RCA added: "It is vital that horse racing and the wider sports/leisure economies have contingency plans to avoid commercially damaging restrictions being re-imposed but equally allow us to host safe events".
The UK government allowed the crowd capacity at Wembley to be raised to more than 60,000 for the semi-finals and final of Euro 2020.
Permitted under the government's Events Research Programme, fans attending Wembley were required to follow a number of strict entry requirements, including having a negative Covid-19 test or proof of full vaccination.
Chief scientific adviser Sir Patrick Vallance said on Monday that nightclubs and other such venues could be "potential super-spreading events" because of crowds in close contact.
A capacity crowd of 140,000 attended the British Grand Prix at Silverstone on Sunday - the largest crowd for a sporting event in the UK since the start of the coronavirus pandemic.
There, all adults had to provide proof of full vaccination or be in possession of a negative test.
In Scotland, Rangers have been given permission to have 17,000 fans present at their Scottish Premiership opener against Livingston on 31 July.
In May, Premier League executive director Bill Bush told the All-Party Parliamentary Group on Coronavirus that a "single magic wand like a vaccine passport" would be "too narrow and too prescriptive".
He said he wanted to work with the Government on a "broader-based" solution to certification at events.
It is understood consultation with Premier League clubs on what approach will be taken to checking fans on entry to stadia next season is ongoing.
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