The attacks took place at Saxilby and Caistor stations in Lincolnshire.

Assistant general secretary of the Retained Firefighters Union (RFU), John Barton, said the action was "deplorable".

Masks were removed and air let out of breathing apparatus stored in fire engines at Saxilby.

The FBU finds such activities abhorrent and would condemn anyone who endangers the life by interfering or damaging fire service equipment

Barry Foster, FBU secretary

Computer equipment and radios used to alert crews to emergencies were also disconnected.

At Caistor fire station pagers used to inform teams that a fire is in progress were also switched off.

The two stations are mostly staffed by RFU crews rather than members of the union which is taking industrial action, the Fire Brigades Union (FBU).

'Had keys'

Lincolnshire Fire Brigade uses a system of master keys which allow firefighters to gain access to all the stations in their county.

Mr Barton said: "It's deplorable really as valuable minutes are lost in repairing and replacing the equipment.

"There's no forced entry. There's no suggestion of a break-in so that would suggest that those responsible had keys."

"I don't think it's a coincidence that both of these stations did not take part in last week's strikes and do not plan to join the next stoppage this week.

'No evidence'

"Maybe it is the FBU, and I can't prove that, but it's certainly a member of the fire service if they managed to get in without breaking in.

The FBU's secretary for Lincolnshire, Barry Foster, said: "There is absolutely no evidence at present to say that it was an FBU member, RFU member or a non-union member.

"But I can say the FBU finds such activities abhorrent and would condemn anyone who endangers the life by interfering or damaging fire service equipment."

Lincolnshire Police said they were investigating reports of burglary and criminal damage at the two sites.