MPs are voting now in the first Commons test of Tony Blair's controversial plans for schools in England face.
The vote is seen as a test of Mr Blair's authority
The Conservatives back the proposals to give schools more control over admissions and budgets. But some Labour MPs are opposed to the package.
It will be seen as a blow to Mr Blair's authority if he relies on Conservative votes to get the changes through.
Government concessions have won over some Labour rebels but others fear a "two-tier" education system.
Some MPs also claim the plans would allow academic selection by the "back door".
Months of wrangling have led to what the prime minister has called a "defining moment" over the Education and Inspections Bill.
Mr Blair is almost certain to win the second reading vote but the spotlight will be on the size of the Labour rebellion.
WHAT'S IN THE BILL?
Parents, businesses and voluntary groups to be able to set up foundation (trust) schools, with control over budgets and admissions
Interviewing pupils and parents to be outlawed
Schools to 'act in accordance with' - rather than just 'have regard to' - admissions code
Schools to set up 'partnerships' with outside groups and 'federations' with other schools
Failing schools given one year to turn around or face being closed and replaced
£30m for councils to raise standards
Under the plans, private companies, faith groups and parents will be given freedom to set up and run "trust" schools within the state system.
A BBC Newsnight survey suggests at least 39 Labour MPs are set to vote against the plans. It only takes 35 rebels for Mr Blair to need opposition support to win the vote.
Education Secretary Ruth Kelly told the Commons: "I will never force any school to become a trust school."
The Conservatives support the bill but - along with Lib Dems and some Labour rebels - will try to sabotage the government's timetable for debating the plans and prolong detailed scrutiny for weeks.
Mr Blair said during prime minister's questions on Wednesday that Tory leader David Cameron was trying to "delay" the plans, despite having "asked me to speed up" before.
But Mr Cameron said: "I will support the government's education reforms, but I don't support the undermining of Parliament."
To allay backbench concerns, ministers have promised a ban on schools using interviews to choose pupils, strengthened the admissions code and told councils they will be able to enter competitions to build new schools.
Mr Blair has said it is a "Labour bill" and Deputy Prime Minister John Prescott this week told backbenchers: "The party and the country would not forgive the Parliamentary Labour Party if the bill was delivered on Tory votes."
Ex-Education Secretary David Blunkett urged Labour rebels to back the bill and not to "snatch defeat from the jaws of victory as we did in the 1980s".
95 Labour MPs backed alternative to government plans
23 of those contacted now support the government but want more concessions later
8 are undecided or away
24 are in the left wing Campaign Group, which is voting against the plans
A further 15 MPs contacted will rebel
Total: At least 39 Labour rebels - it only takes 35 rebels for Tony Blair to need opposition support
Source: BBC Newsnight survey
But Labour's Martin Salter said: "Spinning a set of proposals to curry favour with the Daily Mail and patronising the hell out of Labour MPs is an interesting but ill-advisable political tactic and I trust that ministers have learnt from it."
Shadow education secretary David Willetts criticised the government for not taking radical action sooner, but added: "We on this side of the House are here to praise this Education Bill, not to bury it."
Lib Dem education spokeswoman Sarah Teather said the plans managed to be both timid on reform and contained "hidden dangers".
Tory Ken Clarke, an ex-education secretary, said: "Labour members are never more ridiculous than when they go blue in the face trying to say that the supposed trust schools are not grant maintained schools."