Firefighters have warned of a "threat to public safety" after a financial watchdog predicted a funding gap for the new national brigade.

Audit Scotland has forecast a "potential" annual funding gap of £43m by 2019-20 Scottish Fire and Rescue Service.

This prediction - based on the Scottish Government passing on cuts imposed by Westminster - would see overall spending on the brigade fall by nearly a third in real terms from 2012-13.

The service is already running with far lower costs than those it inherited from the eight regional brigades it replaced in April 2013.

But Stephen Thomson, Scottish secretary of the Fire Brigades Union, said he fear cuts would have to be imposed on the frontline.

Mr Thomson said: "The back office has already been cut to the bone. We are 290 jobs down. Staff costs account for 80 per cent of the total budget so it's hard to see where cuts could come other than reducing the number of firefighters and appliances.

"We believe that is a false economy. Take the Glasgow Art School fire, it caused more than £30m in damage. If we had arrived later, the whole building could have been lost, with all its heritage, and would have to be replaced for, say £300m. That would be more than our annual budget."

Audit Scotland, meanwhile, praised the Scottish Government for the "effective" merger of the eight brigades.

The whole service, before merger, cost £291 million. This financial year it has a budget of £259.2 million. The watchdog, in an official report published today said: "Based on forecasts of UK public sector spending by the UK Office for Budget Responsibility (OBR), we estimate that the service's budget will reduce to approximately £226million by 2019/20 or 31.1 per cent in real terms."

Alisdair Hay, the fire chief, has already signalled difficult times ahead. His brigade last month suggested in a report to the Scottish Parliament that it would have to reconsider its "frontline delivery model".

Audit Scotland acknowledged this, saying such a review would be "major challenge" but urging an urgent long-term financial strategy.

Community Safety Minister Paul Wheelhouse stressed details of the fire budget beyond 2015-16 would have to wait until the new Conservative administration in Westminster had set out its financial plans in July.

He said: "The fire and rescue service will have to deal with future cost pressures at a time when public sector budgets are under pressure."

Mr Wheelhouse said the national brigade response to budget pressures had been "fantastic".

He added: "The situation in Scotland is in direct contrast to England and Wales where, since 2010 over 4,700 fire fighter posts have been cut - more than the number of whole-time fire-fighters in the all eight predecessor Scottish Fire Services at the end of 2012.

"During that period, 39 fire stations across England and Wales have closed - 10 in London alone."

More than half of all incidents attended by the brigade are false alarms, many as a result of equipment failure. Audit Scotland calculates that such faulty alarms cost the service £19m in 2013-14.

Scottish firefighters are dealing with far fewer 42 per cent fewer fires since a peak in 2006-07. Fatalities have fallen by more than 60 per cent in the last decade.

The service boasts that this is partly thanks to a huge increase in home safety visits. Audit Scotland, however, highlighted other causes, such as the decline in smoking and deep-frying.

The watchdog added: "There is an ongoing debate as to whether targeted home fire safety visits will remain the best approach to reducing dwelling fires in the longer term.

"An alternative approach may offer better results, for example focusing on behavioural and cultural changes across society."

That would leave firefighters free to do other things, such as attending medical emergencies. Mr Thomson of the FBU said his union was open to this kind of change "provided we are properly trained and funded to do so".