Fire engines are regularly unstaffed in some areas of Scotland because of an increasing pressure on budgets, the Fire Brigades Union has claimed.
Its Scottish secretary Stephen Thomson told the BBC that a lack of cover meant appliances in the Grampian and Strathclyde areas could not be used.
A spokesman for the Scottish Fire and Rescue Service (SFRS) insisted it maintained "the right resources".
He added that people could rest assured there was "excellent" nationwide cover.
However, Mr Thomson said an "insufficient number" of firefighters in Scotland was becoming a "serious problem".
He claimed that;
- of the six fire engines in Aberdeen, one was regularly unavailable for fire cover
- and in the West of Scotland area four appliances were regularly removed from frontline availability
Mr Thomson explained what he believed were the reasons in Aberdeen: "There is a problem there because I think there is a lack of recruitment and also firefighters are leaving to go to the oil and gas industry that has led to significant shortages.
What that has led to, on occasions, is a frontline appliance regularly being taken out or made unavailable for fire cover."
Of the situation in the Strathclyde area he said: "In the West of Scotland, the former Strathclyde region, there are regularly four frontline fire engines being removed from frontline availability.
"That has a different reason, that is simply because of overtime budget has been overspent so badly because the service believe they can't afford overtime - a different problem although the same outcome."
In April 2013, Scotland's separate eight regional brigades were merged into one national force in a bid to streamline the service and cut costs.
The SFRS made £16m savings in its first year and is on course to making long-term savings of £328m by 2027/28.
The chief officer of the service, Alasdair Hay, told MSPs at the Justice committee on Tuesday afternoon that although savings were "on track" to be delivered he urged "caution" over any further cuts.
The fire service spokesman said that the Scottish public deserved and expected excellent cover, and that was what they were getting.
He added: "The fact that we are a single Scottish service means we have a full nationwide resource and are not limited by the internal boundaries that used to impact regional brigades.
"Crews and specialist resources from across the country can now be deployed wherever they are needed and since the launch of the Scottish Fire and Rescue Service this ability has been a great asset."
The spokesman said that from "time-to-time" a "very small number" of secondary appliances, based at stations with multiple vehicles, could be taken "off the run" to allow firefighters assigned to them to crew the "primary appliances at stations affected by staffing shortages".
He explained: "The choice of which secondary appliances are placed 'off the run' takes account of strategic cover so that every community always remains fully protected.
"There has never been a single occasion anywhere in the country when we have not responded to an incident with the required resources and that will continue."