The controversial education bill agreed by MPs last night was "not the last word on reform", Alan Johnson warned today.
The new education secretary said the government would "push ahead with a refreshed and revitalised radicalism in our schools policy", insisting that this constant drive was what had kept the political left together.
"We'll lose the momentum, lose the argument and may well lose office," he told the Fabian Society in his first major speech since taking office.
His comments may seem surprising given the divisions exposed within the Labour party during discussions on the education and inspections bill, with 69 backbench MPs rebelling at the report stage.
But Mr Johnson was dismissive of those who insisted the pace of reform was too fast, or the changes - which will see the creation of self-governing trust schools - were untested.
"The process of continuing reform over a long period in power may well offend those simple souls who believe that a successful government leads us all to a land of milk and honey over about six years, after which any reform is an admission of previous failure," he said.
"But progressive government is about constant reform and renewal."
A good education system was vital to ensure every child could fulfil their potential and keep Britain competitive, he said, and although standards had improved in the past nine years, the measures included in the education bill were vital.
"The measure for educational achievement is not how much money you put in, but what results you get out. So we need to make sure we match our massive investment with radical reform," he said.
"If we don't, we risk losing the argument for universally provided, publicly funded services, which opens the door for those who want to break up and privatise our public services."
Teaching unions reacted angrily to his comments, however, saying they were not opposed to reform in itself, noting in particular their support for the education bill's proposals to give teachers more disciplinary powers.
"Education does have to adapt and change over time. It cannot stand still," said Philip Parking, general secretary of the PAT.
But he added: "Reforms should change things for the better.The government seems afraid that without momentum it will fall.
"However, if it goes too fast and fails to take teachers, support staff, parents and students with it, it will fall on its face and we will all suffer the consequences of its failure."
Steve Sinnott, general secretary of the National Union of Teachers (NUT), said he was "confident" that Mr Johnson understood the importance of raising pupils' achievement, particularly those from poor backgrounds.
"But the education secretary is wrong to belittle the opponents of the education bill," he said. "The reason the government has had such a tough time piloting its bill through the Commons is because the proposals are profoundly divisive and damaging."