The Spanish prime minister has threatened to snub the weekend Brexit summit unless he receives a public and written guarantee from Theresa May within 36 hours over the future of the disputed territory of Gibraltar.

With May on the brink of formal agreement on her deal with the EU’s leaders, Spain is still insisting it does not have the necessary reassurances that Gibraltar will only be covered by any future trade agreement if Madrid consents.

Downing Street has been accused of acting surreptitiously “under the darkness of night” to insert an article in the deal to include the Rock in its territorial scope.

Emerging from talks with Michel Barnier and diplomats from the other 26 member states, Spain’s EU minister, Luis Marco Aguiriano, said the UK had privately offered assurances on Gibraltar, but that the Spanish prime minister, Pedro Sánchez, wanted to see it in writing before deciding whether to come to Brussels on Sunday.

Sánchez, speaking to reporters in Cuba on Friday afternoon, reiterated that Madrid and London remain divided over the issue. “We still haven’t got sufficient guarantees on Gibraltar. They’re still not sufficient and that’s why Spain is maintaining its veto over the Brexit deal.”

Spain does not have a formal veto over the 585-page withdrawal agreement and the accompanying political declaration on the future relationship, but the EU would be unlikely to go ahead without its support.

Sanchez said that he would attend if acceptable guarantees on Gibraltar were put in place before the summit. Asked whether he would only attend the meeting if a deal was reached in advance, Sánchez echoed his government’s earlier suggestions that the summit could yet be postponed.

In Brussels, Aguiriano said: “The 27 [EU member states] and the commission have offered an agreement package that tries to comply with the demands and objectives of the government of Spain. They have gone as far as they could. They have even improved the political declaration. They have reinforced it.

“And we have a promise, a commitment, from the British government willing to clarify the interpretation of article 184. The clarification that we have been asking for for days.”

Article 184 states that there will be negotiations to define the future EU-UK relationship.

Aguiriano added: “Therefore, we are waiting to see that statement in writing. We have demanded that they make it public before the European council, and as soon as we have that declaration and that commitment in writing, the minister and then the president will have to decide.”

A spokesman for the British government denied Aguiriano’s claims. He said: “Of course we have not agreed to exclude Gibraltar. The prime minister has been consistently clear that we intend to negotiate future agreements on behalf of all territories for whose external relations we are responsible, including Gibraltar.”

Luis Marco Aguiriano (left) with Spanish foreign affairs minister Josep Borrel and Michel Barnier. Photograph: Olivier Hoslet/EPA

With the political temperature rising on the Gibraltar issue, it also emerged on Friday that the EU plans to pile further pressure on May at the Brexit summit by declaring that the post-Brexit negotiations over fishing rights in UK waters would build on the current arrangements hated by the British fishing industry.

A leaked draft statement, due to be published and agreed by the EU’s leaders at the summit, says it expects such a deal to be agreed by July 2020, and that it must protect the current rights of European fishing fleets to exploit British waters.

It goes on to declare that a failure by the British government to come to an agreement could jeopardise any hope of extending the transition period.

Such an extension is likely to be needed by Britain’s government to allow comprehensive trade talks to develop and to avoid the backstop arrangements for Northern Ireland kicking in, which would see a regulatory border drawn between it and the rest of the UK.

The EU’s common fisheries policy is deeply unpopular with the UK fishing industry, which wants Britain to be negotiating access and quota rights as an independent coastal state by the end of 2020. According to recent estimates, 33% of the catches made by the rest of the European fishing fleet are caught in the UK’s exclusive economic zone.

May has insisted “the fisheries agreement is not something we will be trading off against any other priorities”, but the 26-page political declaration published on Thursday makes a clear link between European access to British waters and the terms of a wider economic deal.

The leaked EU statement, to be issued by the 27 heads of state and government, goes even further. The document, which also asserts the EU’s intention to keep the UK “dynamically aligned” to its environment policies, including its climate change targets under the Paris 2030 accord, says the EU “will demonstrate particular vigilance on the necessity to maintain ambitious level playing field conditions and to protect fishing enterprises and their communities”.

It also says: “As recalled in the withdrawal agreement, a fisheries agreement is a matter of priority and should build on, inter alia, existing reciprocal access and quota shares.”