Hundreds of union members are threatening to strike over the council’s plans to privatise IT and back office services - a move members warns will put 850 jobs at risk.

As part of the Keep Tower Hamlets Public campaign, up to 3,000 Unison members in the borough could take to the streets against the town hall’s latest money-saving measures.

They argue outsourcing the work to an outside company is likely to lead to job losses and detrimentally affect workings conditions and pensions.

Unison assistant branch secretary Graham Eddy said: “This means less jobs for the local economy and members’ terms and conditions will be eroded.

“To make matters worse, management have failed to enter into any meaningful dialogue with Unison.”

A lack of accountability and a two-tier workforce where employees doing similar work end up on different contracts could also be a result of the move, the union warned.

After announcing its proposals last month, the council said it is “testing the options” and insisted contracts will not simply be handed to the highest bidder.

Tower Hamlets mayor Lutfur Rahman said: “I’m not in the business of privatising our services to the highest bidder irrespective of the impact on staff.

“I’m looking for a partner who shares my values, who can bring their size and expertise to reduce our costs and who recognise that their best assets are the people that work for them.”

Faced with �70 million of cuts, town hall chiefs said they have no choice but to work with the private sector.

Meanwhile, the union also accuses the council of misleading them on the scale of the changes.

Mr Eddy added: “On May 23 up to 35 bidders attended an open day and were told that a seven-year contract, worth �28million a year, was on offer.”

“This is clearly more than “just IT”, which is what we were told when we met the director of resources.”

Tower Hamlets Unison has drafted in Dexter Whitfield, a leading expert on public sector privatisation, to scrutinise the proposals and advise on alternative options.

The union said it will ballot for a strike which could be “sustained over a period of time” if council chiefs do not enter into talks.

A spokeswoman for Tower Hamlets Council said: “Staff and Trade Unions have been consulted and will continue to be involved throughout the process. The council fully intends for them to have a role in selecting a partner that meets the needs of the council and the local community.”