Radio use on lifeboats is available free of charge
Every mountain rescue team volunteer in Scotland has been asked to write to Ofcom to object to proposed cost rises for use of VHF radio frequencies.
The regulator is consulting on planned changes to the radio broadcasting spectrum which could be introduced from next April.
The Mountain Rescue Committee of Scotland and RNLI have concerns about licence costs.
Ofcom has suggested charities could be offered a 50% discount.
In its consultation document, the regulator said a review was needed to bring about a more efficient use of radio communications which it described as a "finite resource".
But even with the discount, mountain rescue teams and the RNLI said the increase would be significant and put added pressures on funds.
Skye Mountain Rescue Team leader Gerry Akroyd told the BBC Scotland news website the costs involved were "astronomical".
He said: "We are a charitable organisation that save lives and is run by volunteers and are not a business.
"The Mountain Rescue Committee of Scotland has written to Ofcom about it and every individual in every team has also been asked to write letters."
Peter Bradley, RNLI's UK operations manager, said the charity pays a 50% discounted fee of £38,000-a-year for its VHF licence.
Radio use on all vessels is free of charge, however, the costs apply onshore and include lifeboat stations and volunteers' pagers.
Mr Bradley said the new proposed fee would be £260,000 - £130,000 if a reduction was agreed.
He said: "In the current economic climate it is going to be asking a lot of volunteer fund-raisers and the public to find that extra £100,000."
Mr Bradley hoped volunteers and the public would respond to Ofcom's consultation, which finishes on 30 October.