Telecoms giant BT is to ban smoking in all its buildings and vehicles in the UK from next month.
BT is also launching a campaign to help smokers quit
It is believed to be the biggest British company to announce such a sweeping change of policy.
The firm's ban comes into force more than a year ahead of laws outlawing smoking in pubs and other buildings in England, Wales and Northern Ireland.
The firm, which employs 85,000 people in the UK, starts its ban on 26 March when the ban begins in Scotland.
The move has been welcomed by anti-smoking groups which urged other firms to follow suit.
The ban will affect the firm's workers in the UK although it will try to bring in a ban in its overseas premises, but it recognised this may be difficult to enforce.
The telecoms giant believes a third of its workforce regularly smoke.
The company's offices have been smoke free for a number of years but smoking rooms have been provided and workers have been allowed to smoke in BT vehicles.
From next month smoking will be banned from vans and cars being used for business purposes, and smoking rooms closed.
The firm will also be launching a campaign to help smoking workers quit. And some shelters will remain outside buildings for those who still want to smoke.
Dr Paul Litchfield, BT's Chief Medical Officer, said: "We are trying to promote good health among our staff which is why we are offering support to help people give up smoking. I hope this will encourage people to give up."
Anti-smoking group ASH (Action on Smoking and Health) welcomed the move and said it hoped it set "a model" for other employers.
A spokesman said: "There is no need to wait a year for the new law to come into force. Good employers should be acting now.
"Companies are realising that if their staff give up smoking they will benefit in terms of less sickness absenteeism."