Mad cow disease was discovered in herds in Washington state

Japan said it would accept US beef from cows aged 20 months or younger.

The agreement came after three days of talks in Tokyo, and was announced hours before the US Secretary of State, Colin Powell, was due to visit the capital.

Japan was the US' most lucrative beef export market until the trade was banned last December, following an outbreak of mad cow disease.

At the time, US beef exports to Japan were worth well over $1bn a year.

Cattle age

"This is a very important milestone in our returning to normal," Agriculture Secretary Ann Veneman said.

"We have put significant measures in place to further strengthen our already strong food safety system."

US beef should begin appearing in Japanese stores within a matter of weeks, said JB Penn, the US agriculture department's undersecretary for farm and foreign agricultural services.

However, the new rules will only allow a small fraction of US beef products into Japan, as most farmers in the US do not keep records of when their cattle were born.

A joint US-Japanese study will look into whether US methods of dating the age of cattle to within a few days are accurate.

Officials will discuss dropping all restrictions on beef, in July 2005, Mr Penn said.

Mr Powell, is visiting Japan for talks expected to focus on concerns about North Korea's nuclear weapons programme.