Hundreds of comprehensive schools will be allowed to select the brightest pupils and turn away under-achievers as part of the most radical reform of the education system in 50 years.

Theresa May will promise today to let comprehensives apply to choose their pupils based on ability, potentially creating hundreds more grammar schools and abandoning the central tenet of David Cameron’s education policy. The prime minister will also allow the existing 163 grammar schools to expand, while changing the law so that a new generation can open.

The reforms would mark the biggest change to the English education system since the late 1960s when grammar schools began to be converted to become all-ability comprehensives. The creation of new grammars was outlawed by Tony Blair in 1998.