Royal Navy vessels will only be deployed to protect fishing waters surrounding Northern Ireland if requested to do so by the Stormont Executive, the Government has said.
It was revealed the UK had "run live exercises" that involve scrambling "naval vessels to respond to threats of illegal fishing in our soon-to-be sovereign waters" as part of efforts to prepare for a no-deal Brexit.
The Ministry of Defence (MoD) confirmed that four Royal Navy gunboats have been placed on stand-by to guard British waters from EU trawlers if there is no agreement.
Reports also suggest that UK Government ministers are considering beefing up Navy powers in legislation to authorise them to board and arrest fishermen found to be contravening post-Brexit rules.
However, the MoD told the Belfast Telegraph that gunships will only be deployed around Northern Ireland if requested to do so.
"The Royal Navy will only be deployed to assist with fisheries protection and enforcement activities if requested to do so by the Northern Ireland Executive as this is a devolved matter," stated an MoD spokesperson.
"Any request would go through under the usual MACA process (military assistance to civilian authorities)."
The UK Government's move to deploy naval vessels in such circumstances would likely incense EU leaders.
Tobias Ellwood, Conservative chairman of the Commons Defence Committee, called the threat "irresponsible" and warned it would damage Britain's international reputation.
Military officials disagreed, however, with Admiral Lord West, a former chief of naval staff, arguing that fiery past clashes between fishermen in the Channel suggested armed forces intervention could be required.
Brussels has called for the status quo on fishing rights to continue for 12 months in the event of no-deal - a request that appears to have been rejected after Government revealed plans to scale up patrols.
In a speech at a climate change summit on Saturday, Mr Johnson appeared to take aim at French President Emmanuel Macron over the fishing row that has held up a Brexit deal.
Mr Macron is said to have threatened to veto a UK-EU deal after expressing dissatisfaction at the new quota terms being thrashed out for French fishermen.
In his closing remarks, the Prime Minister thanked summit co-host Mr Macron, adding that he knew the En Marche! leader "shares my keen interest in protecting the ecosystems of our seas".
Speaking on The Andrew Marr Show yesterday, the Secretary for Foreign Affairs Dominic Rabb said it is "crazy" of the French to make "aggressive demands" when a no-deal scenario would leave their boats with "zero assured access" to UK fishing grounds.