Workers at outsourcing firm Capita are to stage a nine-day strike after the breakdown of talks aimed at resolving a dispute over pensions.
Members of Unite will walk out from October 28 to November 5 in protest at plans to close their defined benefit pension scheme.
National officer Dominic Hook said: "It is deeply disappointing that Capita staff are being forced into industrial action because of their employer's disgraceful plans to slash the deferred pay that staff will get in retirement.
"Unite suspended planned strikes in good faith in order to reach a settlement with Capita. Unfortunately the company has failed to make a reasonable offer to address the concerns of their workforce.
"Capita's pension proposals will have far-reaching consequences for the retirement of many Unite members. Some staff will lose a shocking 70% of their retirement income."
Unite said the strike will affect Capita's life and pensions service in Birmingham, Reading, Bristol, Manchester, Stirling and Belfast.
Capita said it was in the minority of companies still offering a defined benefit (DB) pension scheme to a "small number" of its employees.
A statement said: "Like many of our peers, we recognised that this arrangement is not sustainable in the long term.
"Following consultation with employees regarding moving to a defined contribution pension scheme, we put forward a competitive phased offer that kick-starts pension contributions and, with Capita providing further payments into the existing DB scheme, safeguards employees' accrued benefits.
"The DB scheme has terms that are above the average offered by both our competitors and FTSE 100 companies in the UK. This competitive offer was not accepted by Unite's members."
Capita said it put forward a "material improvement" to its previous offer during talks at the conciliation service Acas.
Vic Gysin, Capita's group operations and performance director, said: "Unite's decision on behalf of its members is unreasonable and clearly disadvantages our affected employees.
"We will work closely with the handful of clients whose services are delivered by some of these employees to ensure we minimise any impacts from any potential industrial action."