Boris Johnson has warned there are signs of a second wave of coronavirus in Europe, as he defended a 14-day quarantine on travellers from Spain.
The PM said the government had to be "swift and decisive" - and it was up to individuals whether to holiday abroad.
It comes after the Spanish prime minister called the UK's decision to change the rules for Spain "unjust".
Pedro Sánchez said tourists in most regions in Spain would be safer from coronavirus than in the UK.
The UK is advising against all non-essential travel to Spain, including the Balearic and Canary Islands. It also removed Spain and its islands from the list of countries which are exempt from the 14-day quarantine rule.
Meanwhile, Germany has also advised against travel to three areas of Spain.
Speaking during a visit to Nottinghamshire, Mr Johnson said: "What we have to do is take swift and decisive action where we think that the risks are starting to bubble up again.
"Let's be absolutely clear about what's happening in Europe, amongst some of our European friends, I'm afraid you are starting to see in some places the signs of a second wave of the pandemic."
Asked about reports that the 14-day period could be reduced - as reported by the Daily Telegraph - Mr Johnson said: "We are always looking at ways in which we can mitigate the impact of the quarantine, try to help people, try to make sure that the science is working to help travellers and holidaymakers.
"At the moment you have got to stick with the guidance that we are giving, we have given the guidance now about Spain and about some other places around the world.
"I'm afraid if we do see signs of a second wave in other countries it is really our job, our duty, to act swiftly and decisively to stop... travellers coming back from those places seeding the disease here in the UK."
UK 'made an error'
In an interview with the Telecinco TV network, Mr Sánchez said his government was "talking with British authorities to try to get them to reconsider" the decision.
He said the UK had made an "error" by considering the infection rate for the whole country.
He added that "64.5% of the new cases registered are in two territories" and in most of Spain the prevalence of Covid-19 was "very much inferior to the numbers registered in the United Kingdom".
The rate of infection in Spain is 35.1 cases per 100,000 people, while the UK is at 14.7, according to the latest figures from the European Centre for Disease Prevention and Control.
While the outbreak remains under control in many parts of Spain, certain areas - in particular Catalonia in the north-east, which includes the city Barcelona, and the neighbouring region of Aragón - have seen a huge spike in infections.
Data up to 19 July suggested there were lower rates of infection in the Balearic and Canary Islands than in mainland Spain.
Speaking to BBC Breakfast, local government minister Simon Clarke said: "We obviously continue to work closely with them [Spain] and we wish them every success in managing this outbreak, but we've seen a very sharp increase in cases in Spain.
"A 75% increase in cases reported between the middle of last week and the end of last week. That's why we took the action that we have."
Mr Clarke was asked about the risk to holidays to other destinations, following a recent rise in cases in Germany and France. Mr Clarke said the possibility of the rules changing "has to be factored in".
"By all means go on holiday but understand that there is a chance you may be asked to self-isolate upon your return," he said.
On Tuesday, the UK government added Estonia, Latvia, Slovakia, Slovenia and St Vincent and the Grenadines to the list of countries which are exempt from the quarantine rule.
Travellers returning to the UK from anywhere not on the list - including Spain - must now self-isolate for 14 days at a registered address.
People who do not self-isolate can be fined up to £1,000 in England, Wales and Northern Ireland and those returning to Scotland could be fined £480, with fines up to £5,000 for persistent offenders.
Government sources told the BBC that there are no plans to introduce testing at airports, and the priority is to get walk-in centres up and running.
Image copyright Getty Images Image caption In Barcelona, some businesses are quiet as residents are being told to stay at home Image copyright Getty Images Image caption Tourism is important for Spain's economy, with about 18 million Britons travelling there last yearSome travel agents say they are struggling to understand the logic of the UK government's advice.
And MP Chris Bryant, who chairs the all-party parliamentary group for Spain, said: "Why the Canaries - which are further away from Barcelona than Barcelona is the UK - are on the list as well as mainland Spain, I simply don't understand.
"And there are many, many regions of Spain which have much lower infection rates than many areas in the UK. I think this has been terribly badly handled."
Labour said the government's handling of the restrictions had been "chaotic", and urged the government to step in to protect jobs in the travel industry.
"The airline industry and passengers need clarity," said shadow transport secretary Jim McMahon.
Holiday companies Jet2 and Tui were among those to announce sweeping flight cancellations following the UK announcement.
EasyJet, British Airways and Ryanair said they would continue to operate full schedules of flights to Spain, though EasyJet said its holidays would be cancelled for the next few weeks.
Among the thousands affected by the change in travel advice was Tom Clasby, who had checked into an airport hotel near Stansted with his fiancée, their two daughters, and other family members, ahead of a holiday to Majorca.
Mr Clasby, 26, was due to depart at 06:55 BST on Tuesday but now faces having to return home to Bury St Edmunds.
"We're in a situation where we can't do anything yet and I don't actually know what to do. The poor little girls have been so excited for this holiday - it's the second holiday this year we've had cancelled," he told the BBC.
Image copyright Tom Clasby Image caption "We are just very disappointed, the girls will be so upset," said Tom ClasbyAlso affected was Transport Secretary Grant Shapps, who flew to Spain on Saturday despite knowing a decision on the quarantine policy was due.
Mr Shapps said in a statement he would return to the UK on Wednesday in order to complete his quarantine and would return to work as soon as possible.
It comes as a further seven people with coronavirus were reported to have died across all settings in the UK, according to latest government figures - bringing the UK's total number of deaths to 45,759.
In other developments:
Campaigners have written to the health secretary to say a lack of translated coronavirus guidance is jeopardising the safety of non-English speakers in the UK A report from the Commons Home Affairs Committee says stronger action needs to be taken to protect people in asylum accommodation from the virus Emirates becomes the first airline to offer free Covid-19 insurance as it tries to get people flying againHave your travel plans been affected due to the new government advice? Have you returned to the UK and are facing problems due to quarantine? Are you still stranded in Spain? Share your experiences by emailing haveyoursay@bbc.co.uk.
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