The most serious incident was on Saturday morning at PFF Presentation Products in Dockfield Road, Shipley, where fire ripped through the single-storey factory, destroying machinery and putting 18 jobs at risk.

Damage was estimated at around £75,000 and the company, which employs 18 people, now faces an uncertain future.

Arsonists had set fire to some pallets or bins at the premises sometime before 7am. The flames spread to the warehouse, forcing two members of staff, including managing director Craig Knott, to flee.

Firefighers say that around 10 small fires were started in bins and skips in the Dockfield Road area between 5.30am and 9.20am on Saturday morning.

Police made four arrests after receiving a tip-off.

Three of the suspects, men aged 35, 19 and 17, were questioned and later released on bail pending further inquiries. The fourth, a boy aged 14, was released without charge.

Fire investigator Chris Clarke, of West Yorkshire fire service, said 60 firefighters had acted quickly to prevent the fire spreading to other buildings on three sides of the factory.

He said that around 10 fires had been started in bins or skips that morning and urged businesses to be vigilant and to move waste 2-3 metres from premises.

PFF owner Craig Knott said the blaze had been a "kick in the teeth" but he is determined that the business will continue.

"Until we get clearance from the insurers, we are basically in limbo. We can't do anything until they have cleared the site. We are going to have to relocate and we need new machinery."

A police spokesman said there had been a spate of arson attacks in Shipley in the past two weeks.

Detective Inspector John Priestley of Airedale and North Bradford CID, said: "This series of fires has clearly caused concern in the community and we are conducting a number of lines of inquiry.

"After receiving information from our colleagues in the Shipley neighbourhood policing team we acted to make these arrests and are continuing to investigate the matter.

"Anybody who has information or who saw people acting suspiciously in the Dockfield Road area overnight between Friday evening and the early hours of Saturday morning should contact me at Airedale and North Bradford CID on 0845 6060606. Information can also be passed in confidence to Crime Stoppers on 0800 555 111."

In a seprate incident on Sunday evening, suspected arsonists set fire to 16 portable office buildings at a building site for a new school in Killinghall Road, Bradford. The two-storey block of steel containers housed toilets, offices, computers and other equipment for those working on the new Delius School.

As fire crews investigated the cause yesterday they came under a hail of fireworks thrown by young teenagers. Fire investigators said they suspected an arson attack as there was some evidence of vandalism.

Bradford Council officer Kath Tunstall, strategic director for children's services, said: "The fire was confined to the portable buildings being used as the office base for construction work for the new Delius School.

"Although the buildings are in an extremely poor state, no damage was done to the nearby Lapage and Bradford Moor primary schools or any other properties.

"Urgent assessments are being undertaken to minimise the impact of any delay on the building programme which we hope will be minimal."