Transport Secretary Grant Shapps said he is “not aware” of any cases of the Brazilian coronavirus variant in the UK as travellers from across South America were banned from entry.

The ban, which also covers the Central American state of Panama and Portugal – due to its strong travel links with Brazil – and the former Portuguese colony of Cape Verde, came into force at 4am on Friday.

Scientists analysing the Brazilian variant believe the mutations it shares with the new South African strain are associated with a rapid increase in cases in locations where there have already been large outbreaks of the disease.

British and Irish nationals and others with residence rights are exempted from the measures that were backed by the Scottish and Welsh governments, though they must self-isolate for 10 days along with their households on their return.

Mr Shapps described the ban as a “precautionary” measure to ensure the vaccination programme rolling out across the UK was not disrupted by new variants of the virus.

Asked if the Brazilian strain was currently in the country, he told BBC Radio 4’s Today programme: “Not as far as we are aware, I think, at this stage.

“There haven’t been any flights that I can see from the last week from Brazil, for example.”

Dr Mike Tildesley, an epidemiologist who advises the Government on its scientific pandemic influenza group on modelling group, said the UK was late in imposing the travel ban but that it should minimise the risk from the “more transmissible” variant.

“We always have this issue with travel bans of course, that we’re always a little bit behind the curve,” he told BBC Breakfast.

“My understanding is that there haven’t really been any flights coming from Brazil for about the past week, so hopefully the immediate travel ban should really minimise the risk.”

Dr Tildesley said although scientists “don’t believe there is anything to worry about” in terms of vaccine efficacy, the higher transmissibility could mean “people potentially might end up developing severe symptoms more rapidly which could cause more issues with our health service”.

Shadow home secretary Nick Thomas-Symonds said the ban, which includes an exemption for hauliers travelling from Portugal to allow the transport of essential goods, was a “necessary step” but accused ministers of incompetence and “lurching from one crisis and rushed announcement to another”.

Mr Shapps was also forced to defend the delay in requiring negative tests from travellers before they head to the UK from Friday until Monday as a “small grace period” needed to prevent a “repatriation crisis”.

There were tentative signs the national lockdown was bringing infection rates under control in some parts of England.

Cambridge University researchers said the reproduction rate of the virus – known as R – is below 1 in the East of England, London, the South East, West Midlands and Yorkshire and the Humber, meaning the number of cases should be shrinking.

However, they believe it is still above 1 in the South West, North West, North East and East Midlands, indicating case numbers could still grow in those regions.

Meanwhile, the latest figures showed the number of people across the UK to have received the first dose of a Covid-19 vaccine has passed 2.9 million.

Public Health England (PHE) also released data showing infection rates had fallen in most regions of England across all age groups apart from the over-80s, in a further sign that lockdown measures are having an impact.

At the same time, however, the PHE surveillance report noted that there were more people being admitted to hospitals and intensive care units.

NHS England said around one in five major hospital trusts in England had no spare adult critical care beds on January 10.

The time lag between a fall in cases and an impact on the death toll means grim figures are likely to remain a factor for some time.

The latest official figures showed a further 1,248 people had died within 28 days of testing positive for Covid-19 in the UK as of Thursday.

Meanwhile, Boris Johnson is coming under renewed pressure from Tory lockdown sceptics to set out how he will ease the restrictions on people’s liberties as cases come down.

Former minister Steve Baker – who co-ordinated opposition to Theresa May’s Brexit deal – suggested Mr Johnson’s leadership would be “on the table” if there was not a change of direction.

He urged colleagues on the Covid Recovery Group to make their views known to the chief whip, saying: “People are telling me they are losing faith in our Conservative Party leadership because they are not standing up for our values as a party.

“If we continue forward with a strategy that hammers freedom, hammers the private sector, hammers small business owners and hammers the poor, inevitably the Prime Minister’s leadership will be on the table: we strongly do not want that after all we have been through as a country.”

After his comments leaked, Mr Baker later tweeted: “I am clear Boris is the only person to lead us out of these difficulties and I support him in that endeavour.”