New law enforcement powers for Royal Navy Police have come into force.
It means in some circumstances they will be able to board foreign vessels and arrest those breaking the law.
The additional powers for the Royal Navy Police, a specific branch of the Royal Navy with trained military police officers, will only be used if requested by civilian police and apply only to English and Welsh territorial waters.
Tom Sharpe, a retired Navy Commander, told Forces News that essentially the Navy are putting police onto Navy patrol vessels "to ensure that the right fish end up in the right homes".
"The Navy’s been doing this task for many years,” he said.
"They haven't had constabulary permissions to arrest or detain fishermen who are breaking the rules.
"What's changed is that they will now be taking service policemen on to the offshore patrol vessels, who will have the constabulary ability to arrest or detain fishermen who are deemed in breach of the rules," he added.
The regulation was introduced to the Policing and Crime Act 2017 last month when the Ministry of Defence (MOD) laid a Statutory Instrument in Parliament, which allows changes to be made to an act without Parliament having to vote on it.
The changes mean that if there is a maritime incident, Royal Navy Police have more autonomy and can search, board, detain and arrest anyone breaking the law.
"The Government is determined to ensure the security of UK waters from a range of threats under all circumstances," the MOD said.
"No decisions on the use of enforcement will be taken independently by military personnel."
The new powers were announced last year, when no Brexit deal was in place, as part of contingency planning to protect the UK’s fisheries in the event of a no-deal.
While a deal has been reached, the rules around who can fish in UK waters and how much they can catch will be phased in over the coming years.
Having been involved in the protection of fisheries for years, Navy personnel already conduct regular enforcement activity in UK waters and are trained to do so in line with civilian policing procedures.
Mr Sharpe said the fisheries have been "contentious as part of a negotiating package" and while the regulations were complex before, they "are about to get more complex".
"Really by putting policemen on the Navy vessels what... the Navy [has] done there is just provided options and flexibility," he added.
The extra powers come after contingency work for the end of the EU Transition Period showed a potential operational requirement for an extension of powers to Royal Navy Police.
Currently, any interventions at sea are joint operations between the police and military, but the deployment of civilian police officers to a maritime incident may be impractical.
The MOD said "extra powers will give greater flexibility to authorities to act effectively" and Royal Navy Police will only use the powers if requested by civilian police forces.
The extension of enforcement powers will be reviewed in six months' time.