Geoff Adams-Spink BBC News website disability affairs correspondent

One in six people will have a eye problem by the age of 75

The number of people with sight problems is likely to double in the next 20 years, according to the Royal National Institute of the Blind.

The charity is urging the government to spend more on prevention, treatment and support of sight problems.

The RNIB says sight loss is costing almost �5bn a year - a figure that will rise as the population ages.

The government says the reintroduction of free eye tests for the over-60s underlines its commitment.

"The nation is facing a sight loss time bomb," said RNIB chief executive Lesley-Anne Alexander.

"RNIB wants to see additional expenditure of �420m a year to improve treatment."

The organisation says that more should be done for people with conditions like glaucoma and age-related macular degeneration (AMD).

"RNIB is seeking much more than better eye care services - we'd like the government to raise sight loss right up the political agenda," said Ms Alexander.

The Department of Health says it is spending �4m on a number of pilot projects to improve the treatment of glaucoma, AMD and low vision, with a further �27m on diabetic retinopathy.

"Every primary care trust in England will receive an average 8.1% increase in funding per year," a DoH spokesperson said.

"But it is important that decisions about how best to use these funds are made locally, based on local needs."

RNIB would like to see the appointment of a chief eye health officer who would develop standards for health and social care.

And it says the government needs to make people much more aware of the need for regular eye tests.

"They really can, in many cases, be the difference between someone losing their sight or not," according to Ms Alexander.

The DoH said it had collaborated with the RNIB on a campaign to promote regular eye testing.

But it said that this had not been as successful as it had hoped.

"We would need to look very closely at the lessons from this before considering running a similar campaign," the spokesperson said.

The RNIB's warning comes as experts from more than 70 countries gather in London for Vision 2005, a conference that will look at all aspects of sight loss.

Delegates will discuss ways of helping people live with reduced vision and the psychological effects of sight loss.

Some of the latest technology to help people with impaired vision will also be on display.