Tens of thousands of holidays are at risk amid fears Spain could be added to the amber-plus list.
Tourists returning from the Costas would still have to self-isolate even when the EU quarantine rule is axed.
Data expert Tim White warned of the move after NHS Test and Trace figures showed 2.9% of people arriving in England from Spain between July 1 and 21 tested positive for Covid.
READ MORE: August 2 travel rule changes explained - from quarantine to cruises
He said: “More worryingly for Spain is that 3% of the samples had a variant not widespread in the UK.
“I cannot see Spain being put on red but amber-plus is likely – and likely for the islands of Spain, too, because of the very high infection rates.”
However, Paul Charles, head of travel consultancy The PC Agency, claimed infections in Spain were “flatlining at the moment and starting to reduce a little bit”.
He said: “I think it would be a very brazen Government that would tell a million British tourists in Spain at the moment, and Balearics, they would have to quarantine on their way back.
“It would be a really tough decision. I can’t believe they’re going to do that.” Double-jabbed arrivals landing in the UK from Europe and the US will not have to quarantine from 4am on Monday in the latest easing of rules.
But fears over the Beta variant could lead to experts recommending Spain is added to the amber-plus list, The Mirror reports.
It could dash the hopes of UK families hoping for a sunshine holiday.
A July 14 paper from the Scientific Pandemic Influenza Group on Modelling, Operational sub-group, has warned an increase in foreign travel is “of particular concern”.
The next shake-up of the traffic light regime is expected on Thursday.
A Whitehall source said senior No10 figures were split over whether to place Spain on the amber-plus list.
Transport Secretary Grant Shapps urged holidaymakers to wait until an announcement. He told the BBC: “I know it’s very difficult because people will be wanting to go away or have holidays booked or indeed be away.”
Mr Shapps said ministers would be able to clarify rules for Spain when they get details from the Joint Biosecurity Centre.
Spain recorded another 15,111 coronavirus cases on Thursday. And on Friday it was reported the Beta variant in the country has fallen to 2.9% of Covid cases, down from 9%.
There is 0% of the strain in the Balearics and no evidence across much of southern Spain, says analysis seen by The Telegraph.
Meanwhile, No10 is under mounting pressure this weekend to bring forward the date for ending the pingdemic crisis.
Some 689,313 alerts were sent to NHS Covid-19 app users last week telling them they should self-isolate.
The pinging frenzy has triggered staff shortages and hit holiday hopes.
Keir Starmer urged the Tories to switch the date for relaxing quarantine restrictions for the fully-vaccinated from August 16 to August 7. Wales’ First Minister Mark Drakeford confirmed August 7 as the date when double-jabbed adults can escape quarantine if alerted by the app.
Urging No10 to do the same, Mr Starmer said: “We’re seeing a real summer of chaos, you can see the impact it is having on so many businesses, so many sectors.”
Some 80.6% of adults have been double-vaccinated in Wales compared with 71.3% in England. In the UK, 88.4% of adults have had a first dose and 71.8% are fully-inoculated.
PM Boris Johnson insisted August 16 is “nailed on” and Mr Shapps has quashed hopes of an early release.
He told Sky News: “People putting themselves in self-isolation has been a factor in saving a lot of lives.”
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