Four 80-metre (260-feet) vessels have been placed on standby to guard British waters from EU trawlers in case the two sides decide to abandon efforts to secure a free-trade agreement on Sunday.

The development is part of increased contingency planning on both sides of the Channel, and evokes memories of the "Cod Wars" with Iceland over fishing rights in the North Atlantic in the 1960s and 70s.

Prime Minister Boris Johnson has said it is "very, very likely" the talks will fail, and Britain will revert to World Trade Organization (WTO) terms with its largest single trading partner.

European leaders have also been told the chances of a deal are slim with both sides at loggerheads over rules to govern fair competition and fishing rights in British territorial waters.

Deal or no deal, Britain will leave the EU single market and customs union on the evening of December 31, more than four years after a landmark referendum on membership of the bloc.

- 'Robust enforcement' -

Retired Admiral Alan West, who as chief of naval staff and First Sea Lord from 2002 to 2006 was Britain's highest-ranking naval officer, said readying the Fishery Protection Squadron, was a sensible move in case tensions spill over.

"It's absolutely appropriate that the Royal Navy should protect our waters if the position is we're a sovereign state and the government says we don't want any other nation's fishing boats there," he told BBC radio.

The River-class patrol vessels of the Fishery Protection Squadron -- the Royal Navy's oldest frontline squadron with a history dating back more than 500 years -- already enforce UK and EU fisheries law.

Hardline Brexit-supporting Conservative MPs have sought assurances that the navy should be deployed to protect British waters in the event of a "no-deal".

Tory lawmaker Daniel Kawczynski said it would help "prevent illegal French fishing".

But his colleague Tobias Ellwood, a former British Army captain and now chairman of parliament's defence select committee, said that would not go down well internationally.

"We're facing the prospect of our overstretched Royal Navy squaring up to a close NATO ally over fishing rights," he told BBC radio.

"This isn't the Elizabethan times anymore. It's Global Britain," he added, referring to the country's new post-EU foreign policy.

"We need to be building alliances not breaking them apart."

The Ministry of Defence confirmed it has conducted "extensive planning and preparation" for a range of post-Brexit scenarios from January 1, and has 14,000 personnel on standby to help with the transition.

The four offshore patrol boats would be part of a "robust enforcement measures in place to protect the UK's rights as an independent coastal state" that could also include helicopter surveillance.

- Planning difficulties -

WTO terms would mean tariffs and quotas, driving up prices for businesses and consumers, and the re-introduction of border checks for the first time in decades.

That has already raised the prospect of heavy traffic clogging roads leading to seaports in southern and southeast England, as bureaucracy lengthens waiting times for imports and exports.

Transport companies have also warned that EU member Ireland could see import volumes shrink in the event of new customs procedures for goods routed through Britain.

"As an industry we're looking to plan ahead but there's so many unknowns it becomes difficult," said Road Haulage Association director Martin Reid.

Logjams at the Felixstowe container port in eastern England and elsewhere have already raised fears of more to come, and delays in deliveries to shops, businesses and industry.

But the government said they were mainly caused by a "global spike" in demand for consumer goods and the effect of the coronavirus outbreak on shipping patterns and container capacity.