The government is reportedly planning to back new grammar schools, according to Cabinet papers accidentally shown to cameras in Downing Street.
The memo, signed by the Department for Education's most senior civil servant Jonathan Slater, said Education Secretary Justine Greening planned to launch a consultation into opening new grammars.
The blunder emerged when photographer Steve Back, who runs a company called Political Pictures, spotted the document being carried into Number 10.
The proposals would increase the number of selective schools for the first time in a generation.
Slater warns of problems getting the policy through the House of Lords without measures to "avoid disadvantaging those who don't get in".
In the paper, Slater states:
Shadow Education Secretary Angela Rayner said: "The cat is out of the bag: behind closed doors the Tories are planning a return to the bad old days of grammars, ignoring all the evidence which has told us time and again that they do not aid social mobility."
Theresa May backed the expansion of a grammar school in her own Maidenhead constituency while she was Home Secretary.
After she became prime minister, it was reported that May would end the existing ban on new selective schools. However, she has not commented on whether a new wave of grammar schools will be pushed through.
A government spokesperson said in a statement: "The prime minister has been clear that we need to build a country that works for everyone, not just the privileged few."
“We are looking at a range of options to allow more children to access a school that lets them rise as far as their talents will take them.
“Policies on education will be set out in due course and it would be inappropriate to comment further on internal government documents.”
State grammars select pupils at the age of 11 and although they typically produce excellent results, critics point out that they disadvantage children from poorer backgrounds.
The gaffe is the latest in a string of papers being accidentally revealed to photographers sitting opposite Number 10.
In January, an event on Syrian refugees scheduled to take place in Davos featuring the Queen of Jordan was exposed to photographers.