A maximum 10-year jail term for lying about recent travel history has been defended by the government.
Transport Secretary Grant Shapps said the British public "would expect pretty strong action" and the maximum sentence reflects the seriousness of the crime.
It was criticised by former Supreme Court judge Lord Sumption who said lower tariffs exist for sex offences.
From Monday, people arriving in England from "red list" countries must isolate for 10 days in hotels, costing £1,750.
It follows concerns that existing vaccines being rolled out in the UK may struggle to control new virus variants identified around the world.
Meanwhile, Mr Shapps said people should not be booking holidays either in the UK or abroad, and that it was "too soon" for sun-seekers to plan getaways.
"People shouldn't be booking holidays right now - not domestically or internationally," he told the Today programme, prompting the travel industry to warn of another lost summer.
It comes as Prime Minister Boris Johnson told MPs the public would need to "get used to the idea of vaccinating and then re-vaccinating in the autumn" due to new variants.
And, speaking at Prime Minister's Questions, he said the hotel quarantine plan was "measured" and "proportionate".
Writing in the Daily Telegraph, Lord Sumption accused Health Secretary Matt Hancock - who announced the latest measures - of losing his connection with reality.
"Ten years is the maximum sentence for threats to kill, non-fatal poisoning or indecent assault," he wrote.
"Does Mr Hancock really think that non-disclosure of a visit to Portugal is worse than the large number of violent firearms offences or sexual offences involving minors, for which the maximum is seven years?"
Former Tory MP and ex-attorney general Dominic Grieve said the 10-year penalty was "a mistake", "exaggerated" and "entirely disproportionate".
He told BBC Radio 4's Today programme: "The reality is that nobody would get such a sentence anyway, the courts are simply not going to impose it.
"Now I recognise that the government has to put down strict rules and needs to have penalties to enforce them.
"But to suggest that a 10-year sentence is going to result from a false declaration on a form on landing at Heathrow Airport is, I think, a mistake because it is exaggerated, it is not going to happen."
'Lie and cheat'
Failing to quarantine in a designated hotel after arriving from a "red list" country will carry a fine of between £5,000 and £10,000.
The 10-year jail term would be the maximum penalty for anyone found to have falsified their travel history on the mandatory passenger locator form filled in by travellers when they arrive in the UK.
New border measures also require international arrivals to pay for additional tests during their quarantine period.
Downing Street said MPs will not be asked to vote on the plans as the government will be including the restrictions under the Forgery and Counterfeiting Act 1981.
Asked about harsh penalties attached to the new measures, Mr Shapps told BBC Breakfast those who are fined as much as £10,000 would have to "go out of their way to lie and cheat" the new system.
Mr Shapps said the 10-year maximum jail term reflected the "serious" nature of the offence.
"I think the British public would expect pretty strong action" for those who seek to evade hotel quarantine, he said.
Around 1,300 people a week are arriving into the UK from the 33 red list countries - including Portugal, Brazil and South Africa - at the moment, Mr Shapps said.
International travel is currently banned, other than for a small number of permitted reasons, including for essential work, medical appointments and education. Holidays are not allowed.
Mr Shapps confirmed ministers were speaking to other governments about a potential "international system" to check whether travellers have been vaccinated - or tested for - coronavirus.
Downing Street said Mr Johnson will outline a "roadmap" for easing the current national restrictions in the week of 22 February.
England's deputy chief medical officer Prof Jonathan Van-Tam told a BBC News Your Questions special it was "certainly plausible" other countries may insist visitors are vaccinated.
But he stressed mandatory vaccination has never been a policy in the UK before and described confidence in jabs among the public as "super high".
Airlines and travel companies will be legally required to make sure travellers have signed up for the new measures before they depart, with fines for companies and passengers if they fail to comply, he said.
The penalties also include a £1,000 fine for travellers who fail to take the new mandatory tests and a £2,000 fine for failing to take the second mandatory test - along with a 14-day extension to quarantine.
Asked when the travel rules would be relaxed, Mr Hancock said: "We want to exit from this into a system of safe international travel as soon as practicable and as soon as is safe."
The Scottish government said it would go further than England's measures and require everyone arriving by air to isolate in hotels for 10 days.
The Institute for Government think tank said different hotel quarantine rules across the UK add "another layer of complexity" as it warned of "big gaps" in the policy.
Speaking at Prime Minister's Questions, Labour leader Sir Keir Starmer criticised the delay in introducing hotel quarantine and said research showed the UK was lagging behind other nations when it came to border security.
All travellers arriving in the UK are already required to show proof of a negative Covid-19 test and must self-isolate for 10 days.
This negative test must be taken in the 72 hours before travelling, and anyone arriving without one faces a fine of up to £500, with Border Force officials carrying out spot checks.
They must also provide contact details and their UK address. They can then travel - by public transport if necessary - to the place where they plan to self-isolate.
A "test to release scheme" - where travellers from non-red list countries can leave home isolation after a negative test on day five - remains under the new testing rules.
Passengers will be expected to use the gold-standard, and more expensive, PCR tests.
People leaving England will soon have to declare their reasons for travel prior to departure.
Have you recently travelled into the UK? What was your experience at the UK border?