BT is to close a number of its call centres, with the loss of hundreds of jobs, as part of a reorganisation.

Union officials fear that up to 4,000 jobs will be lost under the plans, which could face opposition.

BT has refused to give details of the exact number of job losses, or the call centres to be closed.

It will make a more detailed announcement before the end of March.

BT has more than 150 call centres, including sites in Sunderland and Middlesbrough, where services including directory inquiries, fault reporting, accounts and operators are based.

Pierre Danon, chief executive of BT Retail, said a more modern call centre operation was needed to improve customer service and increase efficiency. The telecoms firm will be spending £100m to create a new generation of "multi-function customer contact centres".

BT said there would be no compulsory redundancies and job cuts would be included in a previously-announced figure of 13,000 across BT Retail.

The Communication Workers Union (CWU) warned it would oppose the plans unless the impact on staff was agreed jointly.

"Creating a smaller network of leading-edge, multi function call centres will inevitably mean the closure of a number of sites," said Mr Danon.

He said several sites were likely to close, but union officials fear there will be widespread closure of call centres.

Cutting down the network was the only way to keep the operations in-house and make dramatic improvements to the quality of customer service, said Mr Danon.

The plan will generate huge savings, which will pay back the £100m investment over two years.

Jeannie Drake, deputy general secretary of the CWU, said more consideration should be given to staff redeployment issues.

"A series of meetings have been arranged over the next few weeks where we will be looking for assurances that the people implications have been given proper consideration. Our whole approach to the company's plans will be dependent on the outcome of those meetings."