School cleaners are fighting a “horrendous” new salary system they say will leave them out without pay for long periods at a time.

Staff at the 26 Tower Hamlets schools affected by management giant G4S’s changes are also opposing plans to make them sign in and out of shifts through biometric hand scanning.

Many feel they should not have to hand over their personal data to employers, according to Unison.

The union says it has “overwhelming support” for strike action, with 90 per cent of all its members in favour in a test ballot.

Cleaners, caretakers and engineers affected by the new system will be without pay from the end of August to the beginning of October, at which point a block-payment system will be brought in.

Jean Lane, Unison representative for the schools, said: “It’s a horrendous system and means two and a half weeks’ pay is always held back as well as their normal month’s pay. That money will be sitting in G4S’s bank account gaining interest when these people desperately need it. The level of anger out there is really high.

“With the biometrics, the idea that they have to hand over their identity in order to get paid is ridiculous.”

Employers have a legal obligation under the Data Protection Act to use biometrics proportionately, meaning the benefits should outweigh the intrusion into employees’ private lives.

The biometrics system has already been introduced in Mile End hospital but Unison says some staff there are not happy with it.

Ms Lane added: “It’s not efficient as staff in the hospital have told us very often the machines don’t work or there are queues to sign in and you lose pay if you sign in late.”

G4S said it wants to bring its several thousand

G4S said the changes will bring the schools in line with its several thousand employees across London and is offering advances in salary for those facing difficulties.

It is meeting employees to discuss their concerns.

A spokeswoman added: “Biometric clocks are being introduced in three schools in Tower Hamlets. The clocks do not record any personal data of staff.”