A former head of the Royal Navy has warned that Britain will become a "laughing stock" if it does not have sufficient patrol ships to enforce fishing rules.
It comes after it was announced that ministers will trigger an exit from the London Fisheries Convention, to start a two-year process to leave the agreement.
Signed in 1964 before joining the EU, the convention allows vessels from France, Belgium, Germany, Ireland and the Netherlands to fish within six and 12 nautical miles of the UK's coastline.
Admiral Lord West of Spithead (above) said that as a "simple sailor" he was stunned by ministers' "amazing complacency". He told peers:
"We will be made a laughing stock if we apply rules but cannot enforce them."
The Labour peer said a number of countries had already indicated they would continue to fish in British waters and added:
"The bottom line is we have very, very few vessels involved in this and they are not properly centrally coordinated."
Lord West has called for a centralised command system to crack down on illegal fishing and for the government to build more boats once the change is made.
Environment minister Lord Gardiner of Kimble assured the House of Lords the UK had a "robust enforcement system" delivered by the Marine Management Organisation and in-shore fisheries and conservation authorities.
But he said the government would need to "review and reflect on the level of fisheries enforcement required" on leaving the EU.
Lord Gardiner said a monitoring system enabled the authorities to know every ship that was at sea in British waters and there were new off-shore patrol vessels being built to help existing ones with fisheries protection.
For the Opposition, Baroness Jones of Whitchurch said that to have sustainable fishing there had to be some co-operation with neighbouring countries. She said:
"This can't be resolved by unilateral declaration. There has to be detailed discussion on the future."
Lord Gardiner said the two-year notice period would allow time to negotiate with "partners and friends in Europe so we have a sustainable fishing industry" and the ability to decide who fishes in British waters.
Liberal Democrat Baroness Parminter urged ministers not to use fishermen as a "bargaining chip" in Brexit negotiations.
UKIP's Lord Pearson of Rannoch said the British fishing industry, which had been "decimated" by the EU's common fisheries policy, will have a "wonderful future" once Britain has taken back control of its waters.
Lord Gardiner said the purpose was to ensure a sustainable future for the UK fishing industry. "It's the first time in 50 years we will have been able to control access to our waters."