A union in Andalusia has announced plans to occupy Gibraltar for one day next month with the goal of denouncing tax havens, dismantling the British military base, protecting small-scale fishing in the area and reclaiming Gibraltar as Andalusian territory.

The decision to take action against Gibraltar on 29 August was made at a recent national assembly, said Andalusian Workers' Union spokesperson Diego Cañamero Valle. Emphasising that this would be a "peaceful protest", he added that it was primarily an action against tax havens. "It's a crucial topic right now in Europe, as there are more than a dozen tax havens where money is hidden, allowing people to dodge taxes."

The details have yet to be worked out, he said, including how many of the group's 20,000 members would participate and how exactly they would get around border security.

"We're going to enter peacefully. We're not going to face off against anyone or break anything," said Cañamero Valle. "It's sincerely an act of peaceful protest in the face of the economic situation that we're living right now. Unemployment is around 25% and even higher in Andalusia. There's little access to credit and no money for public services like hospitals and education. Meanwhile tax havens continue to exist."

Gibraltar has said it is not a tax haven and complies with all European Union tax and transparency regulations.

The Andalusian Workers' Union has made headlines in the past, often for its collaborative actions with Marinaleda mayor, Juan Manuel Sánchez Gordillo, dubbed a Spanish Robin Hood by local media. Together they have occupied lands and turned them over to cooperatives to be farmed and carried out raids on Spanish supermarkets, leaving with trolleys laden with food and school supplies to hand out to families in need in Andalusia.

The group's interest in Gibraltar comes after a year of high tension between London and Madrid over the creation of an artificial reef by Gibraltar. The move angered Spain, which argued that the reef disrupted Spanish fishing boats.

In a move that many saw as retaliatory, Spain imposed tighter controls at the border, leading to long queues for people trying to enter Gibraltar. Spanish authorities said the stricter border controls were necessary to crackdown on tobacco smuggling.

Last week the European commission said that Gibraltar had not violated EU environmental regulations in the creation of an artificial reef, as Spain had previously alleged in various complaints.

• This article was amended on 29 July 2014 to clarify that while the union claims Gibraltar is a tax haven, the territory itself says it complies with all EU tax regulations.