Royal Dutch Shell, Italian oil giant Eni and a number of senior executives at the two firms face trial in Milan on Monday over corruption charges relating to a $1.1bn (£800m) deal for a Nigerian oil block.

The Milan public prosecutor alleges that $520m from a 2011 deal to buy rights to a vast oil block off Nigeria’s coast was converted into cash and intended to be paid to the then Nigerian president Goodluck Jonathan, members of the government and other Nigerian government officials.

The prosecutor further alleges that money was also channelled to Eni and Shell executives, with $50m in cash delivered to the home of Eni’s then head of business for Sub-Saharan Africa, Roberto Casula.

Four former Shell staff members face trial, including Malcolm Brinded, former executive director for upstream international operations, along with two ex-MI6 agents employed by Shell.

Also standing trial are Eni’s chief executive Claudio Descalzi, former chief executive Paolo Scaroni, and chief operations and technology officer Roberto Casula.

Shell, Eni and their executives have denied all charges.

The trial comes after years of campaigning by anti-corruption groups Global Witness, The Corner House and Re:Common, as well as British-born Nigerian campaigner Dotun Oloko.

After investigations by Global Witness and another anti-corruption group Finance Uncovered, Shell admitted in April last year that senior figures knew that some funds from the deal would be paid to companies controlled by former Nigerian oil minister Dan Etete.

Barnaby Pace, an anti-corruption campaigner at Global Witness, said the trial should be a turning point for the oil industry.

“Some of the most senior executives of two of the biggest companies in the world could face prison sentences for a deal struck under their watch.”

Antonio Tricarico, of Italian NGO Re:Common, said: “This case heralds the dawning of the age of accountability, a world where even the most powerful corporations can no longer hide their wrongdoing and avoid justice.”

Eni has said in a statement on its website that the trial “will give the opportunity to Eni to fully defend its position and to provide full evidence of the correctness of the actions taken with respect to the OPL 245 transaction.”

Regarding the allegations against Eni’s CEO, the company has said: “Eni’s board of directors has reaffirmed its confidence that the company was not involved in alleged corrupt activities in relation to the transaction.