The transport secretary said he did not think the Brazil variant was already in the UK "as far as we are aware".
It comes as a further 1,248 people with coronavirus have died in the UK.
Latest government figures on Thursday also showed another 48,682 new cases had been reported.
The UK's new travel ban - which also applies to Portugal and Cape Verde - came into force at 04:00 GMT on Friday. Like variants discovered in the UK and South Africa, it is thought the Brazil variant could be more contagious.