The Government is facing an increasing threat of widespread industrial action over its flagship academy programme.
Teachers at 13 Birmingham primaries are being balloted over plans to turn their schools into academies, two teaching unions announced today.
It is understood to be the first time that teachers in such a large number of schools have been balloted over the issue at one time.
The National Union of Teachers (NUT) said its members in the schools involved had asked to be balloted for action because they are angry that the change in employer that moving to academy status would bring could have a significant impact on their terms and conditions.
It claimed that these primaries are being forced by Birmingham Council to become academies.
NUT general secretary Christine Blower said: "Forced academies have nothing to do with raising standards of education. There is no evidence at all to support the notion that changing the status of a school will transform the educational attainment of its pupils. The majority of the school communities in Birmingham do not support this move. The local authority needs to listen to the people who elected them."
John Dixon, the NUT's head of organising and membership, said: "There is quite a number of schools in Birmingham that are being told by the authority that they want to force them into academy status.
"We are in the process of talking to our members in all of these schools and at the moment, there are 13 where our members have asked to be balloted, there may be others later."
The schools involved are being targeted for academy status because they are failing to meet the Government's "very broad" measures of what schools should be achieving, the NUT suggested.
Mr Dixon said: "It is the first time that so many are being balloted, that's because of the push in Birmingham to try to convert so many. That's why it's such a large issue."
In the past, it has been staff at individual schools that have taken industrial action over a move to academy status, rather than teachers at a group of schools.
Chris Keates, general secretary of the NASUWT, which is also balloting its members in the 13 primaries, said: "Schools in Birmingham and all over the country are now facing unacceptable and unfair pressure to convert.
"This is no way to run an education service. This is not the way to improve educational outcomes. There is no evidence that simply converting a school into an academy raises standards."
Today's ballots come just weeks after the NUT called for further industrial action over academies.
At their annual conference in Torquay, the union agreed to establish a national campaign of local, regional and national action against further academy conversions.
A Birmingham City Council spokesman said that academy conversion is just one of the options being used to improve schools.
"Where we need to intervene we will look at what is best for the individual school rather than a one-size-fits-all approach," he said.
"However, Birmingham has a number of primary schools that have been under the floor standard for a number of consecutive of years and school-to-school support may not result in the rapid improvement needed.
"So for each of these schools with a history of under-performance where a range of other options have not succeeded, such as changed leadership, local authority support and changed governance, we recognise that a radical response is needed - becoming an academy or working with a network of academy schools may be the best way of ensuring sustained improvement."
A Department for Education spokeswoman said: "We are very disappointed that unions are balloting teachers in Birmingham to consider strike action. This will not achieve anything positive for the schools involved and will be hugely damaging to the children's education.
"We believe help from an academy sponsor is the best way to improve schools that are consistently underperforming and letting pupils down. Academies are proven to work - they are popular with parents and they are improving results at twice the national average rate."
Academies are semi-independent state schools that receive funding directly, rather than through a local authority, and have more freedom over areas such as pay and conditions and the curriculum.