The prime minister has given his strongest backing yet for the expansion of grammar schools.

Speaking at an event today, David Cameron said he “strongly supported” good schools growing and included grammar schools in that context.

Mr Cameron is reported to have said: “I strongly support the right of all good schools to expand. I think that’s very important and that should include grammar schools.

“Under this government, grammar schools have been able to expand and that is all to the good.”

It comes as education secretary Nicky Morgan is considering the opening of an “annexe” to an existing grammar school – Weald of Kent Grammar School – in Kent.

Its plans had previously been turned down by former education secretary Michael Gove under the School Standards and Framework Act 1998, which prevents the opening of new selective schools.

Originally the girls’ school applied to open a “satellite” campus, located several miles from its main site, as a co-educational school. Grammar schools can expand if the second site operates under a single governing body managing structure, curriculum, staffing and budget, and if one set of admissions criteria is in place for both sites.

The school’s trust is now applying to keep the satellite site as a single-sex intake, and hopes this will ensure it is compliant with the 1998 framework.

Theresa May last November also backed plans for the opening of a new grammar school in her Maidenhead constituency.

Mr Cameron added: “I don’t want to pre-empt [Nicky Morgan’s] decision, that’s a decision for the Education Secretary to make. But the principle is very clear: good schools should have the freedom to expand.”

Previously, deputy prime minister Nick Clegg told Schools Week said he was against grammar schools and plans to expand.

The Department for Education said a decision about the Kent grammar school would be made in “due course”.