POLICE fear a serial bomber is at large, with farmers and businesses connected with agriculture the main targets.

A 58-year-old farmer in Ripon, North Yorkshire, and a 43-year-old assistant from an estate agents in East Yorkshire needed hospital treatment after devices went off yesterday morning.

Detectives believe the attacks could be linked to another incident in Cheshire a week ago, and the discovery of a device in the mail sent to a North Yorkshire firm just before Christmas.

An animal rights fanatic could be behind the bombs.

The man leading the hunt for the bomber in North Yorkshire, Detective Chief Superintendent Ian Lynch, admitted yesterday there were real fears for public safety.

And the county's Assistant Chief Constable, Peter Walker, added: "None of the victims have been people engaged with animal experiments; they are merely members of the farming community going about their daily business.

"Although we do not wish to spread unnecessary panic or alarm, we feel it is prudent to warn people now to take care when opening their mail."

Agricultural firms in particular are being urged to mark any packages they send clearly and, if they receive any post which appears suspicious, to leave it unopened and dial 999.

Yesterday, a parcel bomb, packed with carpet tacks, blew up as a sheep farmer from Ripon opened his mail.

He escaped with cuts to his face, although the blast was powerful enough to destroy the chair he was sitting on and damage other furniture in the kitchen. Two other adult members of the family escaped unhurt.

The farmer - who has not been identified as police fear he may be targeted again - was taken to hospital for treatment but later allowed home.

However, a woman opening the mail at an estate agents in Partington, near Bridlington, was not so fortunate.

Another device - again packed with small nails - exploded in her face.

She needed treatment for eye injuries at the Hull Royal Infirmary, where her condition was described as comfortable last night.

Frank Hill and Sons, where she works, deals with livestock auctions.

Last Saturday, a six-year-old girl from Congleton, Cheshire, needed hospital treatment when a package she opened detonated, peppering her legs and feet with pins and small ball-bearings.

She was at home at the time but the property is also the address of a pest control business.

And, on December 15, a company based in Masham in North Yorkshire received a bomb through the post.

This time, a receptionist escaped unhurt, but only because she opened the package at the wrong end. Colleagues at Ritchey Tagg evacuated the building until the device was made safe by the Bomb Disposal Squad from nearby Catterick Garrison.

The firm makes ear tags for cattle and sheep, and supplies specialist equipment to vets.

Forensic experts will now be working on the remains of the devices which arrived in yesterday's mail, as well as the previous bombs.

Detectives from the two Yorkshire forces have confirmed they will be comparing notes with officers from Cheshire, as well as the Metropolitan Police in London, who have experience of similar incidents in the past.

However, Mr Walker emphasised yesterday no definite link has been established between the attacks, although it is known that all four bombs were sent in Jiffy bags.

He said: "It is only good fortune that no one has been seriously injured or killed.

"I am extremely concerned that these devices are being sent through the post.

"I find the concept of sending explosive devices in the post totally abhorrent. It is a common terrorist tactic."

There is speculation that animal rights activists may be behind the attacks but, as no one has yet claimed responsibility, the motive for the bombings remains a mystery.

However, it would not be the first time extremists have targeted Yorkshire.

A series of incendiary devices were planted in shops in Harrogate and York during September, 1994, detonating overnight when the premises were empty.

There were no injuries, although damage was estimated to be worth more than a million pounds.

No one has ever been charged in connection with the attacks, timed to coincide and stretching the county's fire service to the limit