A group of former employees have been arrested on suspicion of deliberately starting a devastating blaze that killed four firefighters in a vegetable packing warehouse 18 months ago.

The three men and one woman, all Polish nationals in their 20s, were detained at addresses in Birmingham and Worcestershire suspected of arson with intent to endanger life.

Police officers believe that some or all of the suspects worked at the Wealmoor fresh produce packaging plant at the time of the fire. They were being questioned today by detectives at a Warwickshire police station

It took 200 firefighters to bring the powerful fire under control in Atherstone on Stour, Warwickshire on November 2, 2007. Four of the firefighters died as they struggled to put out the flames making it the deadliest blaze for the fire service in 30 years.

Around 100 rescue specialists, trained for disasters such as earthquakes, needed four days to find the badly burnt bodies of three of the officers who had been trapped inside.

Soon after the fire, police said the inferno was being treated as suspicious but no one had been arrested until today.

This morning’s dawn raids came after an 18-month investigation by a full-time team of 38 police officers and support staff who have carried out hundreds of interviews.

A Warwickshire Police spokeswoman said: “We’ve still got a very large team working on it. It’s a very unusual and very complex case because most of the evidence was burnt in the fire.

“It is believed that at least some of the suspects were employees or former employees at the time of the fire. In other words they did have some kind of connection with the building.”

The fire killed Ashley Stephens, 20, Darren Yates-Badley, 24, John Averis, 27 and Ian Reid, 44, who were called in to extinguish the flames.

An investigation by Warwickshire Police and the Health and Safety Executive into the fire is ongoing, but last year, an inquiry revealed that fire officers were not given enough information before attending the scene. As a result, an improvement notice was issued to Warwickshire Fire and Rescue Service.

Wealmoor, a vegetable packing company that operated the warehouse, is owned by Ratilal Dhanani, whose family fortune is estimated at £5.2 million.

The company has been operating since 1973 and distributes 60 types of exotic fruit and vegetables from 55 countries. It is thought to employ a large number of migrant workers in Britain, some of them reportedly slept at the warehouse in between long-shifts.

A police spokesman said: “Early this morning three men - two aged 25 and one aged 27 - were arrested at addresses in the Handsworth and Smethwick areas of Birmingham and a 26-year-old woman was arrested in the Evesham area.

“They have been arrested on suspicion of arson with intent to endanger life and are in custody at a Warwickshire police station where they will be questioned by detectives.”

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