One of BT’s biggest unions has accused the company of targeting older, long-serving staff in its drive to cut 13,000 jobs.

Prospect, one of the largest unions representing BT workers, claimed the company was using an unfair “out with the old, in with the new” strategy to help reduce costs by £1.5bn.

At Prospect’s annual BT conference in Brighton, delegates passed a series of motions accusing the company of misusing and inconsistently applying its “people framework” plan.

Noel McClean, Prospect’s national secretary, said: “This year’s conference motions instructing us to continue to hold BT to account have indicated that the company is not operating effectively and transparently in its reorganisation despite assurances given last year.

“It is clear that its efforts to cut costs are driven by an ideological ‘out with the old, in with the new’ attack on staff. They have managed to create a hostile environment where loyal, long-serving employees are railroaded out.”

One motion passed at the conference said that while there were a range of dismissal measures an employer could choose to use, “within that context there is a difference between something being legally permissible and morally justifiable”.

BT has already made the first few thousand job cuts of the scheme. Two-thirds of the 13,000 total will fall on its UK workforce of about 83,000. BT has 23,000 staff employed internationally.

“It is clear [BT] places no value on skills and retraining and reskilling its long-term and experienced employees,” McClean said. “Their cheaper replacements, many of whom are starting out in their careers, are expected to do more for less, exploiting their need for job security in this precarious working world.”

BT is shutting 20 of its 50 offices in the UK and moving out of its London headquarters in St Paul’s, where it has been since 1874. The sale of the headquarters is likely to raise more than £200m. BT’s preferred option for a new London base is the One Braham development in Aldgate.

Last week BT’s chief executive, Philip Jansen, unveiled a morale-boosting plan to gift employees £50m in shares each year.

Jansen, who took over from Gavin Patterson in February, acknowledged that staff morale was at a low after a tough couple of years for BT, which have included an Italian accounting scandal and a profit warning that stripped billions off its market value. BT’s shares are trading at their lowest level since 2012.

BT has denied reports that it could cut up to another 25,000 jobs to seek a global staff number closer to 75,000.

A BT spokesman said: “We’ve consulted extensively with Prospect and our employees on the changes we are making to build a better BT for the future. People’s age or length of service is absolutely not a consideration, although in some cases employees have opted for voluntary redundancy and early retirement.”