Heads are being "encouraged" to turn their schools into academies by civil servants who are repeatedly phoning to make sure they sign up as soon as possible, unions claim.
They say schools are receiving unsolicited calls from Department for Education officials.
"We've heard so many stories from heads who say they are being encouraged to go for academy status as soon as possible by civil servants who are ringing around," said Kevin Courtney, deputy general secretary of the NUT.
"If there are rushed decisions we predict many legal challenges by teachers or parents who don't believe they have been consulted enough, especially because the school will be spending public money."
The DfE admitted that schools that have expressed interest in making the move are being phoned by officials, but it claimed that this was only an attempt to be helpful.
Members of heads' union the NAHT and the Association of School and College Leaders have also reported phone calls.
Lindsey Kings, deputy head of Torquay Boys' Grammar, was phoned by Government officials when she registered interest in becoming an academy.
"They asked what we thought about it and if we are going to go for it, and told us all the procedures," she said.
"We were rung only two days after we signed up via the internet."
Unions have also warned that school leaders are being "rushed" into making a decision while being kept in the dark about how much money they will have to spend.
They are demanding detailed information about school budgets from the DfE and have taken issue with the only tool headteachers have been given to make a prediction - an internet calculator.
The "ready reckoner" has been criticised for not taking into account the complexities of education spending. It estimates the extra income an academy could receive by reference to the percentage of the school's funding currently retained by its local authority.
Unions say teachers should use more accurate data before they commit themselves. Schools will receive a one-off grant of pound;25,000 to convert to academies.
"They shouldn't act on this before knowing for sure how much money they will get, and they should be aware of extra costs which will come about from becoming an academy. There are contracts to be renegotiated, services to pay for, plus the legal costs of setting up which we estimate will be at least pound;75,000," said Mr Courtney.
A DfE spokesman said academies would not be better funded than mainstream schools.
"It doesn't give them advantaged funding, just the comparative sum of money headteachers need to pay for all services themselves," he said.
"The idea is for the Government to replicate the money they would have been given when they were under the control of the local authority."
He also denied that schools were being "cold called" to persuade them to become academies.
"The idea is to help them through the process, not to promote; we are not in the business of telling schools they need to do it. Either way we are relaxed."