Life companies including Aviva, Prudential and Royal London are being hit by a two-day strike held by Capita staff.
A Unite spokeswoman says areas impacted include pensions administration, contract centres, finance and IT projects.
The 48-hour strike is part of an ongoing dispute over staff pay – which Unite says is a real terms pay cut for 75 per cent of its members – and follows a strike held in June.
There will be strikes at sites in Reading, Bristol, Bournemouth, Birmingham, Manchester, Glasgow, Belfast and Stirling.
These sites service firms including Prudential, Aviva, Royal London, Abbey Life and Guardian.
In April 88 per cent of Capita Unite members voted to reject a pay deal.
Unite national officer Dominic Hook says: “Unite the union members across the country have been given no choice but to have to stand up against the derisory pay offer made by their employer Capita. It is deeply regrettable that Capita have not returned to the negotiating table to resolve this dispute.
“Capita have thus far failed to recognise that it is the hard work and commitment of their workforce that has brought them another year of positive financial results. The pay deal on offer would result in a real terms pay cut for 75% of staff, this is unacceptable.
“In a nationwide show of strength Unite members are now staging a 48 hour strike, with the ban on overtime continuing. If Capita wishes to resolve this dispute with their workforce they must now bring positive proposals to the pay negotiations and recognise the value of their staff.”