The incoming Spanish government is coming under intense diplomatic pressure to fall in line with EU proposals to ban wasteful fishing practices, after a leaked document showed that the previous administration was planning to derail the plans.

The government must choose between supporting its new allies in the EU, on which Spain's economic future depends, or bowing to its powerful fishing industry.

As Europe's biggest fishing industry, Spain could hold the key to the success or failure of the reforms, which would prevent fishermen discarding edible fish at sea.

But according to a secret government document seen by the Guardian, Spain's previous administration was plotting a last-ditch attempt to bring down the reforms, and allow Spanish fishermen to continue throwing away edible fish as they have been doing so for decades

The document is dated November 2, just weeks before the general election of November 20, showing that lobbying for a continuation of discards was a key policy for the outgoing socialist government even in its dying days.

Hugh Fearnley-Whittingstall, the chef and Guardian food writer who has led the high-profile Fish Fight campaign against discards, said: "Throwing away thousands of tonnes of perfectly edible fish is completely unacceptable – and I'm sure that if the Spanish public knew about the scale of the problem in their fishing fleet, they too would demand an end to discards, and support a radical overhaul of the EU's common fisheries policy."

He said that early in 2012, the campaign, which Europe's fisheries chief Maria Damanaki has credited as an important factor in generating public support for her anti-discards policy, would be extended to the rest of Europe.

Fearnley-Whittingstall said: "We're planning to launch our Fish Fight campaign in Spain, as well as in France, Germany and Poland. We need to get the whole of Europe as outraged about this senseless waste of fish as we are here in Britain. Then perhaps the Spanish government will spend more time working towards developing a sustainable and effective solution to the discards problem, rather than trying to build alliances to derail the proposed reforms."

If the new government reverses the stance of its predecessor, it would mark a major victory in the fight to end discards, which result in European fishermen throwing away – dead – as much as two-thirds of their catches of live, edible fish.

Discards are a perverse result of the current EU fishing policy: whenever fishermen accidentally land fish for which they have no quota, or when they exceed their quota, they must throw part of the catch away. Nearly three-quarters of the EU's fish stocks are now estimated to be overexploited.

As Europe's biggest fishing nation, Spain is in pole position to obstruct or water down the reforms, which are aimed at protecting dwindling fish stocks and which the European Commission wants to finalise in 2012.

David Ritter, oceans campaigner at Greenpeace, said the current quota system was "indefensible", and the leaked Spanish policy document showed the sort of attitudes that must be overcome if radical reform of the Common Fisheries Policy was to succeed. He said: "[This is] a broken system that is destroying Europe's marine environment and the fishing industry that depends on it."

He called on David Cameron to join in the fight against discards. "Cameron needs to give his full support to Richard Benyon [UK fisheries minister] in Europe. Our national interest isn't contained by the M25 – it extends to our coastal communities, and they're under severe threat [if the Spanish government opposes an end to discards]," he said.

There is all to play for in the next few days as the incoming Spanish ministerial teams work out their detailed policy responses. While the country's dire economic state will take precedence, fishing is a major economic and social issue for Spain, home to Europe's biggest fishing fleet in terms of volume, taking up more than a quarter of the EU's total fishing capacity and employing tens of thousands of people directly, and many more indirectly.

The new ministers will be under huge pressure from the powerful Spanish fishing lobby to oppose the reforms. Although many fishermen support ending discards, because they hate the waste of throwing back edible fish to die, some want to continue the practice because it allows them to maximise their profits, by throwing away lower value fish and leaving room in their quota for higher-priced fish.

The extent of fish industry lobbying is shown by the secret document seen by the Guardian. It said that Spain wanted to retain discards as a way of allowing its fishermen to land only the most valuable fish, killing hundreds of thousands of healthy and edible fish in the process.

The document said eliminating discards was "not realistic" and instead opted for a reduction in discards "with a gradual approach" that should be varied according to each region – meaning get-out clauses for large sections of the Spanish fleet.

The document said: "The terms and deadlines [on discards] are not realistic, particularly for specific sections of the Spanish fleet. Spain requests that this discards policy be set out in terms of reduction, with a gradual approach, rigorously planned and over a period no shorter than ten years. Furthermore, this will have to be implemented regionally, taking into account the different causes for discards in different fisheries."

The European Commission wants to change fishing quotas in order to reach a scientifically set "maximum sustainable yield" for all fisheries by 2015. But the previous Spanish government wanted to move this deadline to 2020.

From the document, it is clear that the previous administration in Madrid planned to cite the country's economic woes as an argument for watering down the reforms. But the European Commission has repeatedly said that allowing the exploitation of Europe's seas to continue at the current rate would store up economic problems for the near future, as stocks would decline further and fishermen find their livelihoods in peril as a result.

Maria Damanaki, the EU fisheries commissioner, has repeatedly said that urgent action must be taken for stocks to be preserved. On formally presenting her reform proposals this summer, she said: "Action is needed now to get all our fish stocks back into a healthy state to preserve them for present and future generations. Only under this precondition can fishermen continue to fish and earn a decent living out of their activities."

She added: "It is not too late for the situation to be reversed, but we have now reached a crisis point. Overfishing must cease or there will be no more fish on the plate."

The European Commission has proposed compensation for fishermen, and ways to use the catch more efficiently, for instance by encouraging consumers to move away from the over-consumption of a few species, such as cod and haddock, to a wider range of fish that are currently undervalued. Damanaki has also proposed novel alternatives, such as trial projects for fishermen to sweep up plastic detritus from the seas instead of fishing, and to use their boats for tourism.