Vodafone, the world's biggest mobile phone network operator and parent of Eircell, is to cut up to 650 jobs.
The company, which employs some 10,000 people in the UK, blames a change in the mobile phone market for the redundancies.
It will now concentrate less on building new networks and more on retaining customers and improving services.
Hardest hit will be its headquarters in Newbury, Berkshire, where 215 staff will go.
The other 385 job losses will be spread throughout Britain and will take place in the next few weeks.
The cuts will be spread from executive level downwards.
Vodafone will open negotiations with staff from today on over who is to be made redundant and the amount of severance pay.
The group had delayed announcing the job losses for 24 hours to enable it to inform staff first.
"It really reflects our move away from growing customer numbers towards persuading customers to use more of our services," said a Vodafone spokesman.
PA