Travel to the UK from all of South America as well as Portugal will be banned from 4am on Friday because of concerns over the Brazilian variant of coronavirus, the Government has said.

Transport Secretary Grant Shapps said the “urgent decision” to restrict flights from the nations was taken to reduce the potential spread of the new strain, with experts uncertain how effective existing vaccines will be against it.

But British and Irish nationals and others with residence rights will be exempted from the measure, which was backed by the Scottish Government, though they must self-isolate for 10 days along with their households.

Mr Shapps said it was a “precautionary” measure as the rollout of the coronavirus vaccine programme stepped up across the country.

“We don’t want to trip up at this late stage. We don’t have cases at the moment but this is a precautionary approach,” he told BBC News.

“We want to make sure that we do everything possible so that vaccine rollout can continue and make sure that it is not disturbed by other variants of this virus.”

Along with nations including Argentina, Colombia and Venezuela, the central American nation of Panama and former Portuguese colony of Cape Verde will be included in the ban decided by ministers on the Government’s Covid-O committee on Thursday after Boris Johnson said he was “concerned” about the variant.

Mr Shapps said travel from Portugal was being suspended because of its “strong travel links with Brazil”, but there will be an exemption for hauliers travelling from Portugal to allow the transport of essential goods.

The restrictions also apply to the Portuguese archipelagos of Madeira and the Azores, while arrivals from Qatar and the Caribbean islands of Aruba, Bonaire, Sint Eustatius and Saba will lose their exemption from the 10-day isolation period.

Scientists analysing the Brazilian variant believe the mutations it shares with the new South African strain seem to be associated with a rapid increase in cases in locations where previous attack rates are thought to be very high.

Brazil had already temporarily suspended flights from or via the UK as of December 25 over concerns about the highly infectious coronavirus strain that has been traced back to Kent.

Labour’s shadow home secretary Nick Thomas-Symonds said the ban was a “necessary step” but said it was “another example of Government incompetence – lurching from one crisis and rushed announcement to another”.

Meanwhile, the Government was facing criticism for delaying the enforcement of a requirement for travellers arriving in England to receive a negative Covid-19 test before departure.

Mr Shapps said the rules had been delayed “to give international arrivals time to prepare”.

The requirement for passengers arriving in England by boat, train or plane – including UK nationals – to test negative for Covid up to 72 hours before leaving the country of departure was due to come into force at 4am on Friday.

But it has been pushed back until the same time on Monday, amid concern that guidance on which tests would be accepted had not been published early enough.

Yvette Cooper, the Labour MP who chairs the Commons Home Affairs Committee, described the delay in introducing the rules as “truly shocking”.

Labour leader Sir Keir Starmer said the situation was another “complete mess”.

“Yet again we’ve got a Prime Minister and a Government that’s late, slow and even now, at the 11th hour, they’re putting testing back another few days,” he told reporters during a visit to a mass vaccination centre in Stevenage.

“I think people will be bewildered and they will feel that we’re exposed – there’s a gap in our defences. We can’t go on like this with delayed decisions not being made in a competent way.”

Downing Street defended the delay, with the Prime Minister’s spokesman saying the testing law would come into place on Friday as planned but that a “grace period” would allow passengers “a little bit more time” to get the tests required.

Paul Charles, chief executive of travel consultancy The PC Agency, said: “This saga is yet another lesson to Government that it takes time to implement major changes to border controls and they can’t be rushed through.

“Consumers have been panicking, and tour operators and airlines inundated with inquiries, since the initial leak about the plans.

“The Government cannot keep creating confusion when it comes to travel restrictions – there needs to be clarity to help build confidence in a sector still struggling to rebuild due to the lockdowns.”

Travellers will need to present proof of a negative test result to their carrier on boarding, while the UK Border Force will conduct spot checks on arrivals.

New arrivals who flout the rules will face a minimum £500 fine while the operator who transported them will also be fined.

Passengers will still have to quarantine for 10 days regardless of their test results.

Travellers will have to take an internationally approved test, and guidance released by the Department for Transport said they could include PCR tests, nasal and throat swab tests, which take between 12 and 24 hours to return results.

Loop-mediated isothermal amplification (Lamp) tests, which can return results in two to three hours, and lateral flow tests, which generate results in less than 30 minutes, are also acceptable.

Results can be produced as physical documents or by email or text but must be in English, French or Spanish. Translations will not be accepted.

British nationals attempting to return home who test positive must not travel and must follow the local guidance in their host country, and contact the nearest consulate if they need support.

Scotland is taking the same approach to international travellers and has also delayed the policy until Monday.

Wales and Northern Ireland are expected to announce their own plans for pre-arrival testing in the coming days.