Britain has been quietly increasing its maritime defences in an attempt to prevent a repeat of the “cod wars” of the 1970s once the UK fully completes its departure from the EU at the end of 2020.

With a post-Brexit deal on fishing to be negotiated, maritime authorities are taking on two new inspection ships, according to recent parliamentary answers – and will have more than twice the number of Royal Navy patrol ships by the end of the year.

The Department for Environment, Food and Rural affairs has said that it wants “a relationship with the EU based on friendly cooperation between sovereign equals”, but has ramped up preparations in case talks with Brussels break down.

Britain has so far refused to offer Brussels any promises on access to UK waters, where more than 700,000 tonnes of fish and shellfish are caught by EU trawlers each year, prompting the Irish prime minister, Leo Varadkar, to threaten to block a trade agreement on financial services last month in retaliation.

In a no-deal scenario, the UK would be responsible for patrolling and enforcing the exclusion of foreign vessels in its economic waters, a vast area of 80,000 nautical square miles into the Atlantic and the North sea.

The Marine Management Organisation, responsible for English waters, can obtain a further 22 ships in an emergency, and is also considering whether to take on two extra surveillance aircraft to help cover the maritime area.

It is able to draw on three armed Royal Navy patrol craft supplied by the Fishery Protection Squadron – or “cod squad” – each armed with cannon and machine guns, and a crew of about 45.

But by the end of the year, the number of patrol craft will increase to seven as a new generation of vessels are added. The older ones are being retained, a decision first made by Gavin Williamson when he was defence secretary in 2018.

Defence sources said that armed conflict involving fishing vessels was very rare. During the ”cod wars” of the first half of the 1970s between British and Icelandic trawlers, fishing nets were cut and boats on both sides were regularly rammed – and there were a handful of incidents when ammunition was fired.

But while fishing became a totemic issue during the referendum campaign, with many Brexiters arguing that allowing EU boats unrestricted access to British waters decimated the local industry.

As a result of concessions given by British governments since the 1970s, EU member states take a much greater proportion of the fish in UK waters than the national fleet, which now represents a small part of the UK economy, contributing less than £1bn and employing 11,000 people.

Britain also wants EU boats to obtain a licence to fish around the country from the beginning of next year and for boats from the 27 member states to abide by UK environmental regulations, according to a fisheries bill being introduced by the government.