LETTER bombers struck yesterday for the tenth time since December.

The nail bomb was posted to an agricultural business in North Yorkshire - strengthening police conviction that the attacks are the work of animal rights extremists.

Following an incident last month, a woman, from Patrington, in East Yorkshire, has still to discover if she has been blinded after suffering a serious eye injury when a parcel detonated as she opened mail.

A farmer from Ripon, North Yorkshire, was lucky to escape uninjured the same day when another package exploded as he opened it at his kitchen table.

Again yesterday, no one was hurt when a nail bomb was delivered to a firm in nearby Thirsk. The business has not been identified for security reasons.

Staff became suspicious of the package and called police. They called in Army bomb disposal experts from Catterick Garrison, North Yorkshire, who carried out a controlled explosion.

Last night, police urged other organisations which could be potential targets to be on their guard.

A spokesman said: "Anyone connected with agriculture or animals, either directly or indirectly, should treat their mail with caution."

A company in Masham, North Yorkshire, which makes ear tags and equipment for vets, was the first to receive a letter bomb, shortly before Christmas.

North Yorkshire's assistant chief constable, Peter Walker, is leading the hunt for the bombers, although the attacks have not been limited to the northern region.

A second letter bomb was sent to a farm at Hawick, in the Scottish Borders, and was also found and defused.

A spokesman for Lothian and Borders Police said: "There are strong indications that this is connected with other incidents which have occurred since mid-December."

The Animal Liberation Front has denied involvement in the bombing campaign, but said the devices bear the hallmarks of more extreme groups which have pledged to use violence to further their cause in the past, including the Animal Rights Militia and the Justice Department