Research by the BBC has found that more than 30,000 children went missing from English and Welsh schools for "substantial" periods during the 2014-15 academic year.

Stephen Evans, campaigns director of the National Secular Society, said that the figures revealed the "absolutely shocking extent to which children living in Britain are being denied their fundamental right to education".

"The NSS has been calling on the Government to tackle this issue for many years, and action is now being taken. But it is clear that local authorities and police need to put aside sensitivity about religion and minority groups and act robustly to ensure that children's right to an education is protected. That should be the absolute priority."

The figures were obtained from Freedom of Information requests to local education authorities, sent by the BBC's Victoria Derbyshire programme.

The BBC reported that of the 30,000 children who had been absent for an extended period of time, almost 4,000 "could not be traced by the authorities."

In Bradford alone 321 children could not be found, despite "extensive enquiries". There was of a total of 985 children in Bradford who were "missing" for at least part of the 2014-15 school year.

Figures obtained by the National Secular Society reveal that in the last three years Bradford has launched just one investigation into a possible unregistered school in the borough.

Earlier this year Bradford MP Naz Shah said that the council was in "denial" over the extent of illegal schooling in the area, and the BBC's figures show just how severe the problem is.

At the same meeting in March, one Bradford councillor said the council was "not aware of any" unregistered and illegal schools. Ofsted chief inspector Sir Michael Wilshaw said officials were being "naïve".

In less than five months Ofsted inspectors identified more than 100 illegal schools nationally. But education officials have said there may be as many as 35 unregistered Jewish schools in Hackney alone – more than the number of legally operating Jewish schools in the borough.

A report recently published by the Board of Deputies and the Institute for Jewish Policy Research found that "half of strictly Orthodox boys aged 11-15 years do not appear in the strictly Orthodox school system."

"We are not the first to identify this issue; indeed, the Department for Education is aware of it, and it has been covered by the national press," the report's authors noted.

Mr Evans added: "It is vital that local authorities take the necessary action to identify illegal schools and register or close them. That will account for many of these 'missing' children's whereabouts. Tighter inspection and regulation of home schooling should also be considered.

"All children living in the Britain should be entitled to a decent education whatever their parents' religious beliefs and that right must be protected."

The National Children's Bureau told the BBC that the missing children could be at "serious risk" of religious extremism, FGM and forced marriage.

The charity said that "Some councils do a fantastic job, but unfortunately some councils don't do a good enough job by any stretch of the imagination.

"There shouldn't be one child in the country who isn't in school and can't be tracked, because of the potential risks.

'We know [of some] horrendous cases, of sexual exploitation. We also know about the correlation between missing children and the possibility that they may be involved in forced marriage, and of course, issues relating to young people's involvement in extremist activity."

They also warned that the BBC's figures were probably underestimates.

The Department for Education told the broadcaster that "local authorities and the police have clear powers to take action."