British fishing fleets will face a tough struggle to wring a substantial advantage from Brexit, despite the prime minister’s promises, owing to key concessions in the government’s fishing proposals and the difficulty of persuading other EU member states to give up their current rights in British and shared waters, the Guardian has found.

Michael Gove, the environment secretary, published a white paper on Wednesday setting out the UK’s approach to fisheries after 2020, when the Brexit transition period is likely to end. He hailed “a sea of opportunity … we can take back control of our waters and revitalise coastal communities”.

But fishing experts have told the Guardian the detail of the white paper shows the reality may be much less sunny for the UK’s 11,000 fishermen. One of the key statements hidden in the 60-page white paper is: “We do not intend to change the method for allocating existing quota.”

In other words, the vast majority of the UK’s catch will be handled in the same way, even after Brexit. Currently, three big companies control two thirds of the UK’s catch, while smaller vessels are disadvantaged. Conservation groups say giving more of the catch to smaller vessels would cut overfishing.

“This statement in the white paper means there is no sign of a major change,” said Andrew Clayton of the Pew Trust, which campaigns against overfishing. “It is the status quo.”

Under the EU rules, the UK already has and always has possessed the power to alter the way quota is allocated, and if it wished could make more of the quota available to smaller vessels. Successive governments have declined to do so, partly because of the complexity of changing the allocations, which might face legal challenges from the owners of big vessels, but also out of free-market ideology.

The government’s admission on page 12 of the white paper, that it will not seek to make radical changes to this arrangement even when the UK leaves the EU, shows the limitations in “taking back control” over fisheries policy.

Paul Keenlyside, political advisor at Greenpeace UK, added: “Local low-impact fishing communities in the UK have been hung out to dry for years, and it is disappointing to see the white paper does little to resolve this. If the government really believes fish are a public resource and the rights to catch them a public asset, [it] should give a much bigger slice of the pie to local low-impact fishers. The white paper contradicts this principle.”

Barrie Deas, chief executive of the National Federation of Fishermen’s Organisations, downplayed the issue. “[The government] has made a judgement based on economic, social and environmental criteria, that the existing allocation system should continue but that new quota could be allocated on a different basis.”

Bertie Armstrong, chief executive of the Scottish Fishermen’s Federation, added that big vessels would continue to take the lion’s share of quota after Brexit: “The world cannot be fed from the produce of small boats and garden allotments: a quarter of a million tonnes of mackerel cannot be caught inshore with handlines.”

However, the white paper does not make clear where any new quota could come from. There are only two ways of achieving a bigger catch for the UK, said Ruth Westcott of Sustain, an alliance of food and farming advocates: to repatriate some of the nearly 60% of the UK’s fish stocks currently allocated – in some cases, rented or sold by UK fishermen – to foreign-owned vessels, or to discover new or under-exploited fishing stocks. The latter is extremely unlikely, unless the government is prepared to water down its commitments not to overexploit species.

As for repatriating fishing rights in UK waters currently held by other EU fleets, ministers will face stiff opposition. “This would involve the EU and Norway saying yes they are happy with this, which is a fantasy,” said Westcott. One problem is that almost all of the fish stocks taken by UK vessels are shared with other countries: Defra said the only stock not shared was herring in the Clyde. This means a degree of international cooperation on pursuing stocks of cod, haddock, mackerel and other key species is unavoidable.

European fishing leaders are adamant that they will press their governments and the European commission to keep to existing arrangements, whereby their vessels can follow shoals into UK-owned waters. They argue this makes sense because fish do not respect international boundaries, while overfishing in one area will have a major impact on the sustainability of stocks throughout the species’ range. Esben Sverdrup-Jensen, chief executive of the Danish Pelagic Producer Organisation, said: “We want something as close to what we have now, to the status quo, as possible.”

He also made it clear that the EU industry sees fishing as part of an overall Brexit settlement, in which if the UK wants more fishing rights it will have to give up other economic opportunities elsewhere. “You cannot manage fisheries separately from trade. Any increase in fishing opportunities for the UK must be accompanied by sacrifices in other areas, as part of the overall settlement.”

Again, this is directly contrary to Gove’s aims. He was explicit that “market access for fisheries products is separate to the question of fishing opportunities and access to waters”.

It is difficult to see how the UK will be able to hold this line. Fishing makes up less than 0.5% of the UK’s GDP, and however totemic the industry has become in the Brexit talks, ministers may hesitate before sacrificing other parts of the economy to gain a small advantage at sea.