Pupils in England should take Covid tests following the half-term break, the education secretary has said.

Gavin Williamson stressed that asymptomatic testing would help break transmission chains as pupils return.

He praised a major testing programme that has helped to identify and isolate asymptomatic cases since March.

But there are concerns that more schools are having to send students home, as fears grow about the Delta variant first identified in India.

More than 50 million rapid Covid tests have been taken by students across England since January - and pupils have been encouraged to establish twice-weekly testing routines.

Mr Williamson said the testing programme would have been "unimaginable" a year ago.

"I am hugely grateful to the families, students and all those working in education for their role in making it happen," he said.

The Department for Education (DfE) says only around 1% of state school students in England have been absent for Covid-related reasons each day since mid April.

But there are concerns about the impact of the Delta variant.

The National Education Union said this week that schools in Lancashire should stay shut for a week after half term, and that schools were driving transmission.

Lancashire had 10 areas in the top 20 worst-affected by coronavirus cases in England in the week to 28 May.

The National Association of Head Teachers (NAHT) said the latest data on the Delta variant was "certainly concerning", and that more schools were having to close "multiple classes" or "bubbles" - especially in areas with high case numbers.

Paul Whiteman, its general secretary, said it raised "serious questions about the government's decision to remove some of the mitigation measures in schools last month".

Pupils in England's secondary schools were required to wear face coverings in classrooms and communal areas when in-person teaching resumed in March - but the government removed this requirement on 17 May, saying transmission in schools was falling.

Mr Whiteman said the country "must not sleepwalk into further widespread disruption to education".

"The government must be proactive and use all the provisions of the existing contingency framework to ensure that transmission in schools is not allowed to proceed unchecked," he said.

Recent research for the government found that pupils in England fell behind again in the second lockdown.

The prime minister has promised there will be more money "coming down the track" after the government's catch-up plans were labelled a "damp squib".

Education is a devolved matter, meaning that guidance on testing pupils varies in Scotland, Wales and Northern Ireland.