Jamming of global positioning signals (GPS) during Europe's largest military exercise has been suspended, following complaints from fishermen.
The Royal Navy issued warnings in September and October that GPS in parts of Scotland would be disrupted during Exercise Joint Warrior.
But Western Isles fishermen said the first they knew was when their equipment went offline last Friday.
The Royal Navy said the military would seek to address their safety concerns.
Joint Warrior is held twice a year and jamming of GPS in April drew no complaints, according to the military.
The Royal Navy said all appropriate actions were taken to warn of the disruption during this year's second exercise, including a guide which was issued on 7 September.
The guide gives the locations and timings for the jamming of GPS.
The Scottish government confirmed it received the guide in September and put it on its website, but a spokeswoman added that it was the Ministry of Defence's (MoD) responsibility to distribute the information.
A Royal Navy spokesman said: "Joint Tactical Exercise Planning Staff (JTEPS), who co-ordinate the tri-service exercise, issued a guide to fishing vessels, ferry operators and environmentalists on 7 September 2011.
"This notice gave warning of the jamming operations, the specific date and times they would be happening and the locations.
"A warning notice, called NAVWARN 269, was also issued on 3 October and both Aberdeen and Stornoway coastguards have been transmitting regular warning broadcasts on VHF, notifying mariners that the operations will take place."
The spokesman said temporary jamming was routinely practiced in military exercises and was an essential part of preparation for operations.
He added: "However, in order to be absolutely clear that there are no genuine safety concerns to address, JTEPS have suspended jamming for the remainder of exercise Joint Warrior 112.
"This will provide a period of time to reflect with all the relevant authorities about the conduct of GPS jamming and ensure that all parties are fully aware before the beginning of the next exercise which is scheduled for spring 2012."
Austen Campbell, the skipper of the Stornoway-based fishing boat Ocean Spirit, said crews knew nothing of the jamming until their system failed last Friday.
He said: "We weren't notified about it at all.
"We thought it was a problem with our boat but everyone else started complaining about it. The coastguards were giving out warnings today but it still shouldn't be happening."
Mr Campbell added: "We are losing earnings over it until the exercise finishes. It is putting boats at risk."
Westerns Isles local authority, Comhairle nan Eilean Siar, said the jamming had also been blamed for affecting internet connectivity and mobile phone and satellite TV.
Comhairle leader Angus Campbell said: "Whilst the total effect of the jamming is unclear it is totally unacceptable if the MoD's exercises are causing disruption to island communications networks.
"I will be writing to the MoD to seek clarity on exactly what has gone on here and to seek assurances over future exercises."
Caithness, Sutherland and Ross SNP MSP Rob Gibson said the MoD had compromised the safety of fishing boat crews.
He added: "In the North Minch, distress signals for mariners are effectively silenced because of the GPS jamming.
"They say it's for our defence but at what cost?"
Exercise Joint Warrior runs until 17 October and involves Nato warships, aircraft and ground troops.