Opposition politicians have launched impeachment proceedings against Chile’s president, Sebastián Piñera, over possible irregularities in the sale of a mining company, after new details about the deal were revealed in the Pandora papers.

Lawmakers cited an “ethical duty” to hold the president accountable for the alleged irregularities in his involvement in the controversial Dominga project.

Earlier this month Chile’s public prosecutor’s office said that it would open an investigation into possible bribery-related corruption charges and tax violations linked to the sale, which was completed in the British Virgin Islands.

The move is the latest blow for centre-right Piñera as he approaches the end of a turbulent four-year term. Presidential and legislative elections are due in November, with polls suggesting leftwing candidates are likely to gain ground.

The Pandora papers – the biggest trove of leaked offshore data in history – revealed new details of the controversial mining deal.

Piñera’s family sold their stake in the Dominga mine project in 2010 to his close friend and business partner, Carlos Alberto Délano. The Pandora papers investigation found evidence to suggest that the third installment of the payment contained a clause requiring the government not to strengthen environmental protections in the proposed area for the mine in the north of Chile.

Jaime Naranjo, a leftist lower house lawmaker and one of the drivers of the impeachment proceeding, said Piñera had “openly infringed the constitution … seriously compromising the honor of the nation”.

Chile president Pinera faces impeachment after Pandora papers leak | Chile | The Guardian