At least 170 extra part-time firefighters are needed in Sussex to support full-time crews and maintain cover.

The Fire Brigades Union (FBU) claims it means sometimes as many as ten fire appliances in East Sussex are not available for 24-hour action.

While there are enough full-time firefighters, the lack of part-time, or retained, firefighters is causing concern.

Retained firefighters have ordinary day jobs but carry pagers so they can respond to 999 calls. They have to be able to reach the fire station, board an engine and be dressed for an emergency in four minutes.

Steve Huggins, FBU chairman in East Sussex, said: "While we may be up to strength with full-time people, the problem is with retained crews.

"The consequence is that regularly ten appliances are not available throughout the day."

Andy Coulson, of the West Sussex FBU, added: "Having less back-up from retained crews puts more pressure on full-timers.

"If a retained crew in, say Petworth, is not mobilised within four minutes then full-time cover is required from another station."

The retained duty system is unique to the fire service. Retained firefighters work on an on-call basis for up to 120 hours per week.

The 120 hours is seen as one unit and up to three people will provide cover for one unit.

They are typically paid a £2,500-a-year retainer plus fees for emergency callouts of between £7.56 and £14.70, depending on qualifications.

However, East Sussex Fire and Rescue Service is more than 75 units short of full cover and West Sussex estimates it is 100 short.

Littlehampton fire station, currently staffed by part-time crews, may soon become full-time because it is getting so many calls.

The FBU says its 14,000 members who work the retained duty system nationally are seriously overstretched and estimates 3,000 retained firefighters should be recruited nationwide.

FBU general secretary Andy Gilchrist said: "The shortage is acute and getting worse.

"In many areas a considerable number of fire engines are unavailable every day because of firefighter shortages.

"We need a national campaign to raise awareness and properly funded recruitment campaigns."

Senior Divisional Officer Dick Ashley of the East Sussex brigade agreed there was a shortage of retained firefighters.

However, he said the problem was not critical, adding that shortages were felt during the day in the smaller communities where many people travelled to London or outside the locality to work.

He said: "This means there are less people around during the day, although the evening and weekend shifts are not such a problem."

He said Pevensey was now up to full strength following a successful campaign by the local full-time fire station.

He added: "Local needs are different so I don't know how useful a national rather than a local campaign would be."

From July, retained firefighters have been paid pro-rata the same as those who are full time.

West Sussex Fire and Rescue Service spokesman Gary Towson said that due to the firefighter shortage, some retained rural stations were out of action.