A 33-year-old man died after he swallowed an "unknown substance" as he fled police.

His death in Becks Road, Keighley, yesterday has sparked a probe by independent investigators.

The man collapsed as he ran away from police. He was seen swallowing the substance at the same time as he fled.

He became unconscious and despite a battle by officers to resuscitate him, he was pronounced dead at the scene, said a West Yorkshire Police spokesman.

The Independent Police Complaints Commission has been called in to investigate the circumstances.

The spokesman said officers were investigating a drugs-related incident and were in Becks Road when they spotted the man.

The spokesman said: "Upon seeing the police, the man ran off along Becks Road and in doing so, consumed a quantity of an unknown substance, subsequently becoming unconscious.

"Attempts were made by officers at the scene to resuscitate him and paramedics were called. The man was subsequently pronounced dead at the scene."

A post mortem took place yesterday and more toxicology tests are to be carried out to determine cause of death. His identity has yet to be released.

The incident had been referred to the IPCC by the West Yorkshire Police professional standards department.

The circumstances of the incident were being discussed by IPCC officers and police officers also in the afternoon, the spokesman added.

Police sealed off both ends of Becks Road at its junction with West Lane and Oakworth Road while scenes of crime officers carried out checks.

The incident happened at about 10.25am in an area where there are two large mills and close to North Beck, which runs through the town.

Deaths are normally referred to the IPCC if police are in contact with a person when he or she dies or have had recent contact with that person.