The windscreen was shattered by the missile

The warning follows an attack on a paramedic crew in County Durham.

A slab of concrete shattered the windscreen of an ambulance as it was returning from an emergency call near Spennymoor.

Now crews and union officials have warned they will boycott areas where they believe attacks are likely.

The two-man paramedic crew, employed by the North East Ambulance Service, narrowly escaped injury when the missile landed on the ambulance after being thrown from the Spennymoor bypass.

Ambulance driver Steve Tate and his colleague Simon Morris, 34, were returning to their Durham City base after ferrying a woman with breathing difficulties to Bishop Auckland General Hospital when they came under attack.

As they drove along the A688 Spennymoor bypass at 1920 BST on Saturday the lump of concrete hit the windscreen.

Mr Morris, spotted the missile and ducked in anticipation of the attack.

Police escorts

Mr Tate, a married father-of-two, was unable to swerve the vehicle to avoid the concrete block but managed to bring the ambulance to a halt.

Union bosses say four in every 10 crew members can expect to suffer physical attacks in any given year.

In the North East in 2003 a female paramedic took early retirement after two separate assaults.

Now crews say they are close to boycotting areas where attacks are likely or have happened in the past.

They are also considering asking for police escorts when attending attack black spots.

Mr Morris said: "We were both very shaken after what happened.

"It seems to be well-to-do kids doing this as well as those others that you might expect in traditional run down areas."

A spokesman for the health union Unison said MPs were to be lobbied in an effort to change the law to make it an automatic criminal offence to assault an NHS staff member.

As the law stands members of staff are required to make a formal complaint before action can be taken.