Tony Clowes died because of a blockage

Detectives had already made details of three cases public. At a press conference on Friday they announced that a fourth incident was now being linked to the others.

This involves a 65-year-old man who was being prepared for a routine operation at the Royal Bournemouth General Hospital in Dorset last November.

He came to no harm, and an internal investigation carried out at the time ruled it was accidental.

First case

The police investigation was originally triggered by the case of Tony Clowes, from Dagenham, Essex, who died on July 18 at Broomfield Hospital, Chelmsford, Essex, while under anaesthetic.

He had been taken to hospital after injuring a finger in a bike accident.

However, he died from brain injuries after the supply of oxygen to the face mask he was wearing was cut off.

Police have given no specific details of how the blockages were caused in any of the cases they are investigating.

But it is understood that in each case a foreign object was found inside the tube at the point where it connects to a face mask.

Officers stressed anaesthetists across the country have been warned to double check the equipment before using it.

Police are investigating an incident at the Royal Bournemouth Hospital

Essex Police assistant chief constable John Broughton said: "We have always said that this inquiry would consider the possibility that the blockages could have occurred by accident or were deliberate.

"In the light of everything we now know it is our view that it is becoming less and less likely that they were accidental."

A senior investigation officer from Dorset will now work with officers from Essex and Hertfordshire to investigate the Bournemouth incident.

Agency staff

Police will investigate the role of agency staff at all four hospitals.

Dr Gina Radford, from the Department of Health, said the safety of patients was a top priority.

She said: ""I would like to reassure everyone, especially those who have just had or are about to have an operation under a general anaesthetic that everything is being done to make sure that blockages cannot occur to anaesthetic breathing systems.

"All hospitals have been reminded by the Medical Devices Agency how important it is to check breathing tubing before it is used for each patient."

The two other cases being investigated are:

• A policeman in his forties who was treated under anaesthetic for a urological condition at Basildon Hospital, Essex on August 2. The man suffered no ill effects.
• A 57-year-old woman who was being anaesthetised for a routine operation at Watford Hospital, Hertfordshire, on April 26. She made a full recovery.