The bosses of two of the world's biggest multi-national corporations were convicted by a jury of "ecocide" at the Supreme Court yesterday for destroying global ecosystems.

They were put on trial under international laws which establish ecocide alongside genocide, war crimes, crimes against humanity and crimes of aggression, as the most serious in existence.

The chief executives may have been actors, the corporations fictional and the trial a mock-up, but the circumstances surrounding the so-called "crimes" – the destruction of ecosystems during both the Gulf oil spill and the mining of crude oil in Alberta – are real. So is the call for a new law protecting the natural world, placing ecocide among the most heinous crimes known.

Both bosses, of Global Petroleum Company (GPC) and Glamis Group, were convicted on charges of ecocide relating to oil extraction in Canada, while one was acquitted of charges relating to the Gulf spill.

"Companies cannot be given a licence to spill and kill as long as they clean up the mess," said Michael Mansfield QC, appearing for the prosecution yesterday.

Experts witnesses said the Gulf ecosystem had become a "dead zone" after the Deepwater Horizon platform disaster last year.

Mr Mansfield claimed deaths and injury to more than 4,000 birds had been caused during oil spill, for which GPC accepted responsibility.

Controversially, though, the proposed law would place criminal responsibility on the respective CEOs Messrs Bannerman and Tench personally, rather than on the firms.

Christopher Parker QC, for the defence, told the jury to "keep a sense of perspective" when comparing ecocide to war crimes and called his clients "scapegoats extraordinaire".

Proposals to declare attacks on the natural environment an international crime against peace began in earnest in 2008 when launched at the United Nations by British lawyer Polly Higgins. She is seeking to pressure governments to vote for her proposals if they are accepted by the UN Law commission.