Spanish prime minister Mariano Rajoy has rejected what Catalonia’s government says is a final chance to negotiate before it pushes ahead with plans to stage a referendum this autumn on independence for the northeastern region.

The pro-independence Catalan administration has been campaigning for Madrid to agree to a Scotland-style, pacted referendum. However, Mr Rajoy insists he will not negotiate or allow any such vote, on the grounds that it would be unconstitutional.

Rajoy wrote to the Catalan leader that the referendum plan was “a serious threat to coexistence and constitutional order”

This week, as Mr Rajoy and Catalan regional premier Carles Puigdemont had strongly worded exchanges, their failure to find common ground was highlighted, increasing speculation that Spain’s territorial crisis is coming to a head.

On Thursday, Mr Rajoy wrote to the Catalan leader that the referendum plan was “a serious threat to coexistence and constitutional order”.

That was in response to a letter Mr Puigdemont sent him the previous day calling for negotiations to settle the “terms and conditions” of an independence referendum. In a television interview, Mr Puigdemont said his letter represented a “last” offer to the Spanish government to sit down and talk.

The Catalan government has committed itself to staging the binding referendum by the beginning of October and is expected to announce the date and the exact question in the coming weeks.

On Monday, the Catalan premier riled the Spanish government by giving a speech about his proposed referendum in Madrid City Hall. “We are the representatives of a people who want to express their future at the ballot box,” he told those present, as police restrained anti-independence protesters outside. “We’re not a passing fad, or an illness, or the result of a mental disorder.”

Unilateral move

El País newspaper has reported that the Catalan government would unilaterally declare independence if it is not allowed to stage the referendum, according to the terms of a so-called “disconnection law” it has been preparing. Mr Puigdemont’s government denied the veracity of that report, saying that the details published were from an earlier draft Bill, which has since been changed.

However, the leaked details of the Bill have added to tensions, and one senior central government official was quoted in the media as saying that Madrid “will do whatever is necessary to prevent [the referendum]”, adding that “the state, democratically speaking, has a lot of power”.

Several Catalan nationalist politicians have gone on trial in recent months for helping stage a nonbinding referendum in 2014

Spain’s complex political situation has fuelled the uncertainty. Pedro Sánchez’s victory at the weekend in the primary of the main opposition Socialist Party handed him a second spell as leader and is seen as potentially having an impact on the Catalan issue. Mr Sánchez has a notoriously poor personal relationship with the prime minister, meaning they could struggle to offer a united unionist front to the nationalists.

Several Catalan nationalist politicians have gone on trial in recent months for helping stage a nonbinding referendum in 2014, which favoured independence but saw a low turnout due to its illegal status. Polls show Catalans are virtually split down the middle on secession, although a clear majority would like a negotiated referendum.