The former Labour Party leader Neil Kinnock has attacked the Government's school reforms and urged the Prime Minster to scrap his plans for independently run "trust" schools.

Lord Kinnock addressed a 200-strong rally of people opposed to the plans in the House of Commons last night. He said it was the first time he had broken with the tradition of supporting the Labour leadership since he resigned from office in 1992. But he said he believed the "trust" schools - which would have control over their own admissions policies - would create an "unjust" system of state schooling within the UK.

To applause from Labour backbench MPs, parents and teachers' leaders, he said: "Whoever wrote down, or said in a political manifesto or White Paper, that the right way to progress in the future for our children is that we should empower schools to choose whom they admit?" Lord Kinnock, whose attack will increase the pressure on the Education Secretary, Ruth Kelly, said the proposals would give the schools "the power to pick pupils, enrich inputs and guarantee outputs". They would allow schools to achieve "Chelsea positions within league tables".

"Does the country need yet another form of schooling boutique that lacks accountability to the community?" he asked.

Many backbench MPs have signed up to an alternative white paper to the schools reforms - insisting that a code of practice to cover admissions and outlaw any further selection is made legally binding on all schools.

There were signs that Downing Street may be prepared to compromise with the rebels once a report by the Education Select Committee is published next week. The report will demand legal restrictions on schools admissions policies.

Angela Eagle, a member of Labour's national executive who was also at the meeting, said there were so many potential rebels that it was "politically impossible" for the reforms to get through the House of Commons.

She said 91 Labour MPs were supporting the counter-proposals, which she and the former education secretary Estelle Morris had drawn up. But she added: "There are others who will not sign up to our document because it's too soft."

She also warned that opposition to the plans in the Labour Party outside Parliament was even stronger than among MPs.