MPs will soon vote to decide whether Merseyside's grammar schools are free to expand – without a vote by local parents.

Labour has triggered a vote by a Commons committee after condemning a new school admissions code – stripping parents of the right to object – as "expansion by the back door".

Now Stephen Twigg, the West Derby MP and the party's education spokesman, has written to every Liberal Democrat MP, urging them to join forces to defeat the new code.

And he said: "Privately, many Lib Dems have concerns about what the government is doing. If they vote with us, we will have a majority."

Liverpool Blue Coat School is among England's 164 surviving grammars, which currently face strict rules designed to stop them cherry-picking pupils, and taking funding, from neighbouring comprehensives.

There are also six in Wirral; West Kirby girls school, Calday Grange in West Kirby, Wirral boys and girls schools, both in Bebington, Upton Hall School and St Anselm's College, in Birkenhead.

The rules preventing expansion are due to be scrapped on February 1, also removing – most controversially – "the duty to consult locally and the ability to object when a school increases its admission numbers".

Mr Twigg has used a parliamentary mechanism known as "praying against" a motion to force a vote in a Commons committee of between 15 and 20 members.

He said: "I want local schools to attract pupils with a range of abilities.

“I don't agree with dividing children up at the age of 11. If grammar schools are allowed to expand, they will inevitably attract more of the academically able to go there, so that local comprehensives will have a less mixed-ability intake.

"Parents should have a say, yet Michael Gove, the Education Secretary, is taking away their ability to object. This is expansion of grammar schools by the back door."

Mr Twigg pointed out that the Lib Dems had long been opposed to the expansion of grammars - while, before the general election, David Cameron ruled out building new grammar schools.

However, he insisted his move was not the thin end of a wedge that would see Labour, when back in office, attempt to kill off the surviving grammars.

Mr Twigg said: "I am not moving away from our existing policy, which is that there will only be change if parents vote for that in a ballot."

Merseyside's grammars are not believed to have put forward any plans for expansion, but a grammar in Kent has announced plans to set up a "satellite" school.

Ministers have argued that places at popular schools have been "rationed" for too long, denying parents real choice.