Glasgow and Dublin are at growing risk of being cut from the list of hosts for this summer's European Championship.
With fewer than 100 days until the delayed Euros, Uefa wants each of the countries staging matches to submit their plans - including for the return of fans - by a deadline of 7 April.
Organisers say they still hope to stage matches at the 12 venues as planned.
But the Scottish and Irish governments are yet to provide assurances whether fans will be allowed back by June.
And Uefa is understood to be increasingly concerned they may have to strip Glasgow and Dublin of their matches.
Both Hampden Park and the Aviva Stadium are due to stage four games.
According to one source with knowledge of tournament planning, the position on fans in Dublin and Glasgow is currently "very bad", with the respective governments "taking a much tighter approach to Covid than other cities".
On Wednesday, Scotland's Health Secretary Jeane Freeman said that being part of the Euros was "a really big deal" for her country.
But she added that fans being able to attend depended on "all of us abiding by the rules", cases falling, and the take-up of vaccines.
The Scottish government said: "In response to the Covid situation, all 12 host cities are considering a range of planning scenarios, including the scale of supporter attendance at stadiums and fan zones.
"Partners are in regular contact with Uefa, who will review scenario planning with each of the hosts in April before confirming final proposals."
It is understood that the next few days will be crucial and that the Scottish government is aware of the need for clarity.
The Scottish FA says it will "remain in constant dialogue" with Uefa as Scotland looks forward to a first major men's tournament for 23 years.
"We also note Uefa's re-stated commitment to holding Euro 2020 across the 12 European cities, with no other plans being pursued," it stated.
Last week, the Football Association of Ireland said it was still planning for fans to be present in Dublin but admitted it would only remain a host venue if it could guarantee spectators would be permitted.
Irish government restrictions on fans will remain in place until at least 5 April.
The British government has said up to 10,000 spectators will be permitted inside English grounds from mid-May - and unlimited numbers from 21 June.
Earlier this week, Prime Minister Boris Johnson said England could stage more than the seven matches currently being played at Wembley, if Uefa needed it to step in.
The Football Association is understood to be willing in principle to stage additional matches in England if asked to do so by Uefa, but sources suggest there has been no such request to date.
'It's looking bleak' - analysis
BBC Scotland sports news correspondent Chris McLaughlin
Given some of the conversations I've had, things at this stage are looking pretty bleak. I don't think the Scottish government is of a mind to budge, to take any chances on pressure that may well come from Uefa. It just thinks it's too early.
Come June, we may well be in a position to have fans inside stadiums, but Uefa wants assurances now. I don't think the Scottish government is of a mind to take a chance - as it sees it - on giving those assurances. I think it might well be just too late and, at the moment, it's looking bleak.
'Uefa need 30-35% of stadium capacity to turn a profit'
Speaking on BBC Radio 5 Live's Euro Leagues podcast, German football expert Raphael Honigstein said he expected more venues to be unable to host games.
"I think there will be a whittling down and cities will withdraw," he said. "Next month is the deadline for Uefa to get guarantees that [the grounds] can provide enough access for Uefa to turn a profit.
"They need 30-35% of stadium capacity to turn a profit and it will come down to that. I'm not sure even a place like Munich, where all the Germany games are, are in a position to say 'we will be fine in June'.
"In that 30% of capacity, Uefa do not envisage travelling fans and I think that will be the case, even at Wembley.
"It's less of a problem than if it was all held in one country and if the Champions League is anything to go by then Uefa have a degree of flexibility. It's more doable and less of a nightmare than if it was scheduled to just be in Germany or France for the whole five weeks."
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