BBC News website disability affairs correspondent
People in the UK are losing their sight as a result of glaucoma despite treatment being available, according to a report by a leading charity.
Regular eye testing is the only way to detect early glaucoma
The Royal National Institute for the Blind (RNIB) says more than 200,000 people have gone blind needlessly.
The organisation is launching a campaign to raise awareness of glaucoma and of the need for regular eye tests.
Report author Steve Winyard says 40% of vision can be lost before someone realises that anything is wrong.
"People need to be aware that glaucoma does not necessarily have any symptoms," he said.
"Once you do have the symptoms, it's too late."
Glaucoma is the name for a group of eye conditions in which the optic nerve is damaged at the point where it leaves the eye.
The RNIB report - Tunnel Vision - shows that most people are unaware of the condition and what steps to take to prevent it resulting in blindness.
One in five people questioned had never heard of the condition, and only 3% knew that it could have no symptoms.
More than 80% of people were unaware that anyone specifically at risk of glaucoma was entitled to a free eye test.
"The simple message to everyone is, have regular eye tests," said Mr Winyard.
"An eye test can save your sight, and this is even more important to people over 40 and those with people with a close relative with glaucoma as they are at high risk."
The RNIB also says that people of African origin are four times more likely to develop the condition.
The government is being urged to fund a campaign to promote the importance of eye tests in order to catch the disease in its early stages.
By the time Mandy Rotchell found out that she had glaucoma it was too late to save her sight.
Forty-seven-year-old Ms Rotchell, from Rugerley in Staffordshire, was adopted.
You can have a free eye test if you...
Are over 40 with a close relative with glaucoma
Have diabetes or glaucoma
Are over 60
Are under 16 (or under 19 if in full time education)
Receive certain benefits
Get vouchers for complex lenses
Are registered blind or partially sighted
Her biological mother had glaucoma but failed to tell the authorities that her daughter could be at risk.
"I urge everyone, even people with no history of glaucoma in their families, to have regular eye tests," she said.
"The thing with glaucoma is that once your sight has gone, it's gone - you can't get it back."
The RNIB has enlisted the support of newscaster Sir Trevor McDonald, and writer and comedienne, Jenny Eclair, to draw attention to its campaign.
Next month the organisation will take a roadshow to 13 cities in the UK to provide people with advice and information about glaucoma.