U2 frontman Bono has revealed he has suffered from glaucoma for many years, prompting his continual use of dark glasses.

The star said he has had the condition - a build-up of pressure in the eyeball which can damage the optic nerve and lead to blindness if not treated - for around two decades.

Many had assumed his ever-present sunglasses - even indoors - were a rock star affectation, but he explained during a recording of Graham Norton Show for BBC One that they are to help with his vision problem.

Glaucoma can make the eyes more sensitive to light, causing sufferers to use dark glasses to alleviate difficulties.

Presenter Graham asked whether the singer ever removes his shades, to which Bono replied: "This is a good place to explain to people that I've had glaucoma for the last 20 years.

"I have good treatments and I am going to be fine."

He added: "You're not going to get this out of your head now and you will be saying 'Ah, poor old blind Bono'."

Early diagnosis can mean further sight deterioration can be prevented - with laser treatment, eye drops and surgery used to stop it worsening - but the damage to the optic nerve cannot be undone.

The Irish band were on the show to promote their new album, Songs Of Innocence, which was released commercially this week after previously being given away to half a billion iTunes customers, a controversial move which upset some people who said they did not want it automatically added to their music libraries.

Speaking about the furore, Bono told Graham: "We wanted to do something fresh but it seems some people don't believe in Father Christmas.

"All those people who were uninterested in U2 are now mad at U2. As far as we are concerned, it's an improvement."

The album is expected to go into the top five this weekend, but will be the group's first album since Achtung Baby in 1991 which will not debut at number one in the UK chart.

:: The Graham Norton Show is on BBC One at 10.35pm tonight (October 17), with other guests Robert Downey Jr, Stephen Fry and Robert Duvall.