The planned nine-day strike is due to begin from Oct. 28. Last month, members voted to take strike action in a ballot, although an earlier strike was called off while last-minute talks took place.

Capita said on Monday it put forward a "material improvement" to its offer to the small proportion of its workers who were still in its defined benefit pension scheme.

The trade union rejected this, the British outsourcing firm said.

In June, Capita had told staff of significant changes to the defined benefit pension arrangements, which the union said would result in a "massive cut" in retirement income.

Capita was among a minority of companies still offering a defined benefit pension plan, in its case to about 4 percent, or 2,920, of its 73,000 strong workforce.

The company said on Monday it was "surprised and disappointed" on behalf of its affected employees that the union had rejected outright the enhanced offer without consulting its members.

It said because Unite had chosen to reject the offer on behalf of its members without consulting them, it was now unable to go forward with its improved offer and would have to revert to the previous offer.

(The story was refiled to correct reference to an earlier strike in paragraphs 2 and 3)

(Reporting By Justin George Varghese in Bengaluru. Editing by Jane Merriman)