LONDON – Britain is the fastest growing market for the booming Internet adult pornography business, with one in four adults, including 1.6 million women, downloading images each month, a study suggested.

A survey by Nielsen NetRatings for the Independent on Sunday found that more than nine million men — almost 40 percent of Britain’s male population — used pornographic websites last year compared with an estimated two million in 2000.

One in four men aged 25 to 49 — 2.5 million — have visited an adult website in the last month alone, the analysts’ survey found.

Some 1.5 million women used Internet pornography in the last 12 months, up from one million in the previous year, it added.

Men and women spent an average 40 minutes each month looking at pornographic websites while half of all couples watched pornography on the Internet together.

Meanwhile, more than half of all children — 60 percent or seven million — have come across pornography on the Internet while looking for something else, the newspaper said.

Relationship agencies were quoted as saying that they had seen a rise in the number of couples attributing their problems to use of Internet pornography.

“For many women, the reaction is exactly the same as if they discovered their partner is having an affair,” Christine Lacey, a senior counsellor for the Relate agency said.

“They may not be having sex with someone else but the effect is the same if it is detrimental to their marriage.”

The IoS said the British porn industry is now estimated to be worth about one billion pounds (1.45 billion euros, 1.85 billion dollars). British web users also look up the word “porn” more than any other on search engines.