The cuts will mean the firm is axing up to 1,000 jobs after previously announcing plans to cut 600, said the Communication Workers Union.
The jobs will be lost from call centres in Bury, Glasgow, Leeds and Preston Brook, near Warrington, said the union, which is already balloting its members in the company for strike action over outsourcing of work to Capita, job losses and job security.
The company has told staff that the original plans to cut 500-600 jobs on a voluntary basis will be at the top end of the scale with 600 people taking a package to leave by July 6, said the union.
In addition, the company announced that it will cut a further 400 jobs with people leaving by December. Remaining staff will be transferred to Capita in July, with up to 3,800 people affected by the outsourcing and redundancy plans, said the CWU.
Andy Kerr, CWU deputy general secretary, said: "We're dismayed and disappointed at this latest announcement of job cuts from O2 at such short notice. The outsourcing transfer date is less than three weeks away and the goalposts appear to be moving daily. This is not how a decent, responsible employer should behave.
"These job cuts will mean the permanent loss of decent jobs in these towns and cities. We remain gravely concerned for the future job and site security of people affected by outsourcing to Capita and we have further talks aimed at securing the best assurances we can for people's jobs, terms and conditions. We're urging all members to vote yes in the current ballot."
An O2 spokesman said: "These are not compulsory redundancies. They relate to our well-publicised commitment of offering discretionary voluntary redundancy, the terms of which have proved an attractive option for a number of people expressing a wish to leave and allowed them the opportunity to choose the path which suited their personal circumstances. We're now going through the final stages of letting those who applied know what the next steps are.
"The extension of our partnership with Capita means that the transfer of our people will see no change to where they work and their terms and conditions such as their salary will be protected. The deal also offers them a guarantee of job security for two years, something we alone as Telefonica UK could never offer. Capita has a proven and growing reputation in customer contact management and can offer many opportunities to people who are looking to grow their career in customer service."