The contract to manage O2’s customer management services commences from 1 July.
This takes Capita's total of major new and extended contracts to over £2bn this year.
As such, the company has said that it now expects to achieve greater than anticipated growth this year, and “currently has visibility of at least eight per cent organic revenue growth”.
The growth forecast contrasts with guidance the company gave in February for lower margins.
It said that margins dipped in 2012 and warned that it expected a further fall this year to levels seen before 2010 and 2011.
Paul Pindar, chief executive at Capita, said: “O2 is a brand renowned for its innovative and bold approach to delivering services and, as a result, has a loyal customer base.
“Capita will design an enhanced service around customers’ evolving needs, introduce new capabilities quickly and ensure that customers’ experiences continue to be excellent.”
Capita added that final negotiations with O2 were ongoing and that the contract would be one of the largest in its history.
Earlier in May, the firm posted a 20 per cent fall in the value of contracts won during the first four months of 2013 compared to 2012, but said it still expects to deliver strong growth.