Fishing ministers are once again tempted to make decisions based on short-term economic benefits rather than long-term sustainability.
Fishing vessels may still be illegally discarding fish overboard in defiance of a European Union ban, putting stocks at risk, peers have warned.
The EU landing obligation aims to put an end to the throwing of dead fish back into the sea, which took place because fishermen were catching species they did not want or were not allowed to take, as part of efforts to conserve stocks.
Rules to prevent 1.7 million tonnes of fish a year being discarded were brought in following a campaign led by celebrity chef Hugh Fearnley-Whittingstall, with requirements that vessels would land everything they caught.
But six months after the rules took full effect, they seem to have had little impact and the UK government does not know the extent to which they are being complied with, the Lords EU Energy and Environment Sub-Committee said.
The committee of peers warned in a previous report in February that the UK was in a position to implement or enforce the rules preventing discards when they came into force in January.
In the inquiry for that report, the committee heard significant concerns that the ban could have an impact on the industry, infrastructure and ports.
Concerns were raised that fishermen could run out of quota for one fish stock, even though they had quota remaining for others, and would have to stop fishing early in the year, while storage facilities and supply chains would struggle with more undersized fish being landed.
Neither of these things have happened, the Lords said, with only small quantities of fish that would have been discarded being landed, and little evidence of fishing vessels being "choked" by a lack of quota.
This suggests fishermen were continuing to discard fish illegally, a new report by the committee warned.
Continued discarding of fish could cause serious damage to stocks, which raises concerns the Government believed illegal discarding was still taking place and did not know the extent of compliance with the rules, the peers said.
The latest report calls for the Government to mandate the use of remote electronic monitoring of all vessels in UK waters after Brexit, such as CCTV cameras, GPS tracking and sensors to monitor fishing gear use.
The Government must also track improvements in gear that is more selective, to avoid catching undersized fish, in the UK fleet, and work with other member states and the Commission to ensure bycatch reduction plans are implemented quickly.
The committee also raised concerns that the scale of the exemptions to the ban that have been granted undermines the aim of the landing obligation to prevent overfishing and protect fish stocks and the marine environment.
Lord Teverson, chairman of the sub-committee said: "Good progress has been made in recent years to improve the health of fish stocks in EU waters.
"But now it seems that fishing ministers are once again tempted to make decisions based on short-term economic benefits rather than long-term sustainability.
"Unless the discard ban is properly implemented and enforced the UK's fishing industry could in the future find itself with nothing left to fish."
An Environment Department spokesman said: "This government is fully committed to sustainable fishing, including ending the wasteful discarding of fish.
"We continue to work with the industry to promote awareness of the new requirements and have stepped up enforcement to ensure fish caught are landed and accurately recorded. Once we leave the EU, we will have the flexibility to introduce other measures to ensure the sustainability of our fish stocks."
Emily Beament is the Press Association environment correspondent.