Catalonia’s vice-president has warned the looming verdict in the trial of 12 separatist leaders could trigger the formation of a regional “government of national unity” of pro-independence parties and those in favour of an officially agreed referendum on self-determination.

Speaking as Catalonia prepares to celebrate its national day (Diada) on Wednesday, Pere Aragonès said such an alliance would help increase the pressure on the Spanish government to find a political solution to the Catalan issue.

A dozen separatist leaders, including Aragonès’s predecessor, Oriol Junqueras, are on trial at Spain’s supreme court over their alleged roles in the failed push for regional independence in October 2017.

Nine of them – including Junqueras, the former speaker of the Catalan parliament Carme Forcadell, and two influential grassroots activists, Jordi Cuixart and Jordi Sànchez – are accused of rebellion, which carries a prison sentence of up to 25 years. Other charges include sedition and misuse of public funds.

Notably absent from the trial has been the former Catalan president Carles Puigdemont, who along with some colleagues fled into self-imposed exile abroad after the then government of Mariano Rajoy responded to the unilateral referendum and subsequent declaration of independence by using the constitution to assume control of the region.

With a verdict expected next month, pro-independence Catalans and those who view the prisoners’ treatment and possible sentences as unnecessarily harsh are planning to use the Diada on 11 September as another show of support and defiance.

Aragonès said while the verdict could prompt legal appeals on human rights grounds and unleash “massive, peaceful civil demonstrations”, it would also yield an “institutional” response.

“Our first option would be for what we call a government of national unity made up of all of us who want a political, rather than penal, solution – all of us who want a referendum, freedom for the political prisoners and the return of the leaders in exile,” he said.

“We could join together in government to force the state to open political negotiations.”

Aragonès said such a government could include pro-independence parties – his own Catalan Republican Left (ERC), Together for Catalonia and the far-left Popular Unity Candidacy – as well as the so-called Commons, an alliance of leftwing parties including the local branch of Podemos, which favours an independence referendum agreed with the Spanish government.

He also said a regional election could be held in an effort to increase the pro-independence bloc in the Catalan parliament.

“But that’s not our plan A,” Aragonès said. “Our plan A is to reinforce the governing majority to pass the 2020 budget and secure the backing of all those who believe that the solution lies in voting, not prison sentences.”

Catalonia is fairly evenly divided on the question of independence. Pro-independence parties have never managed to win 50% of the vote in the regional parliament, and the October 2017 referendum held in defiance of the Spanish courts and constitution had a turnout of about 43%.

Aragonès, who described the Rajoy government’s heavy-handed efforts to stop the referendum as “a laboratory study in how to suppress political dissidence in western Europe”, said the prisoners’ plight had attracted the support of many people who opposed independence.

“Solidarity with the political prisoners is far greater than the support for Catalan independence: there are a lot of people who, despite not being pro-independence, have been against the pre-trial detention and who view any custodial sentence passed on social and pro-independence leaders as a clear mistake,” he said.

The ERC, which governs Catalonia alongside Puigdemont’s Together for Yes party, has opted for a markedly less confrontational approach than its coalition partners. Puigdemont and his successor, Quim Torra, both favour keeping tensions high with Madrid in order to maintain the pro-independence movement’s waning momentum.

Aragonès acknowledged the divisions within the independence bloc but said they were smaller than the differences between the Socialist party of the acting prime minister, Pedro Sánchez, and the anti-austerity Podemos, which are still trying to form a national government after the general election in April.

Sánchez, who toppled Rajoy’s government in a no-confidence vote last year, has shown himself far more willing to listen to the Catalan government than his predecessor.

But he has made it clear there will be no independence referendum and the country’s constitution – which stresses “the indissoluble unity of the Spanish nation” – must be respected.

Aragonès conceded there was no fast track to Catalan independence.

“The people who lead [the ERC] – and I’ve been a campaigning member for 20 years, since I was a teenager – know that this is a historical process,” he said.

“When it comes to historical processes, you have to build up your forces and it’s not a lineal business. It’s not an administrative process of step one and then step two. It’s a complicated business. You can’t just arrive at independence at 6am tomorrow morning, however much you might want to.”

Last year, about 1 million people gathered in Barcelona to use the Diada as a platform to renew their calls for Catalan independence and demand the release of the jailed leaders.

Although the annual celebrations commemorate the fall of the city at the end of the Spanish war of succession in 1714, in recent years they have been used by pro-independence groups as a show of strength.