A new law to manage UK fisheries post-Brexit is a missed opportunity to change an "unfair" quota system, according to assembly members.
The UK Fisheries Bill will, if passed, set the rules over commercial fishing in the UK after Brexit.
But the environment committee said the bill fails to address the issue of the quota and will stop the Welsh fishing industry from developing.
The UK Government said the committee's claims are "simply not true".
In its report, the climate change and environment committee said Wales is entitled to 1% of the UK fishing quota, under the agreement used to share out the fishing allocation.
The quota refers to the rules regarding the amount of fish that can be caught in the seas.
The committee received reports of Welsh fisherman having "to stand on the quay, unable to go to sea to catch fish, because they don't have the quotas", while watching vessels from other nations taking the fish.
Fishermen called the current rules "unfair".
The committee also said the Welsh Government has not done enough to secure improvements in Wales' quota share.
"Leaving the Common Fisheries Policy will provide a unique opportunity for the UK and Wales to rethink their approach to fisheries policy," said Mike Hedges, chairman of the committee.
"In order to take up these opportunities, it is crucial to address Wales' current quota allocation, which we believe is fundamentally unfair.
"We are deeply disappointed that the UK government has decided that this issue will not be addressed as the UK leaves the EU. Unless this matter is revisited, the benefits for Welsh fisheries arising from Brexit will be marginal."
The Welsh Government said: "The Fisheries Bill is not the mechanism to take forward detailed negotiations between UK administrations, or between the UK and the European Union, on issues such as quota share.
"We continue to press the case around quota shares with the other UK administrations as part of separate discussions."
The department for environment, food and rural affairs said: "It is simply not true to say the Fisheries Bill doesn't deliver for the Welsh fishing industry. The bill creates more powers than ever before for the Welsh Government and the National Assembly for Wales.
"Our priority is to negotiate a fairer share that will benefit the whole of the UK, and new legislation about quota shares is not necessary."