Plans for the daily testing of secondary school pupils and teachers instead of isolation are now on "pause", the Government has said.
A joint statement from Public Health England (PHE) and NHS Test and Trace said the balance between the risks and benefits of a daily testing programme in schools was now "unclear".
The decision comes after the new Covid variant was found to have higher rates of transmission and a higher secondary attack rate, increasing "the risk of transmission everywhere, including in school settings".
In December, ministers said secondary school pupils and teachers would be able to have daily lateral flow tests for a week if they have come into close contact with someone who has tested positive for the virus.
That would mean children could continue their education at school rather than entire classes or year groups being sent home to self-isolate for 10 days .
But on Wednesday morning, Downing Street officials said NHS Test and Trace and PHE had "reviewed their advice", with the decision made "in light of the higher prevalence and rates of transmission of the new variant".
They said further evaluation is required to make sure that daily testing "is achieving its aim of breaking chains of transmission and reducing cases of the virus in the community".
Other parts of the testing plan in schools – including two tests a week for teachers and two tests at the start of the school term for pupils – are still due to go ahead when schools reopen, officials said.
A Government spokesman said: "We are… pausing daily contact testing in all but a small number of secondary schools and colleges, where it will continue alongside detailed evaluation."
The statement from PHE and NHS Test and Trace, published on Wednesday morning, said: "In light of this changing situation, we now recommend that the rollout of daily contact testing within schools is paused, other than for schools involved in further evaluation.
"This will enable the further detailed evaluation of changing circumstances including, potentially, lower infection rates and modelling work required to understand the benefits of daily contact testing in the this new phase of the pandemic.
"Schools should continue to test their staff regularly – twice weekly where possible, in line with recommendations for other workforces that need to leave the home to work – and test pupils twice upon return to school, as has been the case since the start of January."