Staff at outsourcing group Capita in Birmingham will carry out six straight days of industrial action next month in a row over pensions.
The affected staff are employed on contracts in the city for financial services groups Phoenix, Royal London and Friends Life as well as IT projects.
The vote was carried out among hundreds of members of trade union Unite and also affects contracts in Reading, Bristol, Manchester, Stirling and Belfast.
Capita claims that "only a handful" of staff in Birmingham were involved in the vote but it refused to confirm an actual number.
Capita announced in June that it planned to close its current defined benefit scheme and transfer staff to a defined contribution scheme which, the trade union claims, will leave some employees 70 per cent worse off in retirement.
In its industrial action ballot, 95 per cent of members voted for strike action on a turnout of 72 per cent.
Unite national officer Dominic Hook said: "The disgraceful plans by Capita to slash the deferred pay that staff will get in retirement is utterly unacceptable.
"Capita's pension proposals will have far reaching consequences for the retirement of many Unite members.
"Some staff will lose a shocking 70 per cent of their retirement income.
"Unite members want to know how a highly profitable company can undermine the morale and loyalty of hard-working staff by proposing to remove their defined benefit pension while at the same time paying off a failing chief executive.
"There is no justification for Capita to treat its workforce in this manner. The extremely high vote in favour of strike action shows how strongly members feel about this.
"Capita must urgently rethink these pensions proposals in order to prevent industrial action."
The union is calling on the companies which outsource contracts to Capita to intervene.
Former chief executive Andy Parker left the role this earlier this month after three years in charge and his replacement is yet to be announced.
The six days of industrial action will commence on October 5.
A statement from Capita said: "We are in the minority of companies still offering a defined benefit pension plan to a small number, some four per cent of our workforce, of our 73,000 employees.
"Their accrued benefits in the existing defined benefit pension are protected and the new defined contribution offer is above the average terms offered by both our competitors and FTSE 100 companies in the UK.
"We are disappointed that these employees, some 550, are to support Unite's strike action.
"We have plans in place to ensure that any potential disruption to our clients' services is mitigated."