The UN is to stage a rare donor conference on Wednesday in a bid to raise the $80m (£65m) necessary to prevent an ageing oil tanker off the west coast of Yemen exploding and causing an environmental disaster potentially four times worse than the Exxon Valdez spill near Alaska in 1989.

The money is needed to offload more than 1.14m barrels of oil that have been sitting in the decrepit cargo ship, Safer, for more than six years because of an impasse between Houthi groups and the Saudi-backed government over ownership and responsibility.

Previous UN mediation efforts over the potentially lethal byproduct of Yemen’s civil war have failed, partly because the Houthi rebels that now control the capital, Sana’a, have not been able to agree terms for UN-commissioned engineers to board the ship. The Houthis have regarded the ship and its lucrative cargo as their possession and a bargaining chip in the negotiations with the Saudi- and Emirati-backed forces.

Expert engineers and environmentalists have warned the ship is an unexploded timebomb capable of causing an ecological disaster. UN estimates suggest that if the ship’s cargo is unleashed into the Red Sea, more than 200,000 fishermen would lose their jobs and $20bn would be required for a clean-up operation.

The deck of the FSO Safer in 2019, indicating the lack of basic maintenance for several years.

But under a new agreement, laboriously negotiated over six months by the UN and Dutch diplomats, an international donor conference will aim to raise the required $80m to offload the light crude oil. The plan is the brainchild of the UN resident coordinator and humanitarian coordinator for Yemen, David Gressly, who claims it has the support of the Saudi-backed and Houthi-backed governments.

A memorandum of understanding was signed by the Houthis on 5 March that allows the UN to transfer about 1.1m barrels of oil from the vessel, which is stranded 8km off Ras Isa port on Yemen’s Houthi-held west coast. The oil would be transferred to a secure vessel which would remain in place. A new tanker would be purchased for the Houthis within 18 months to replace Safer, thus providing them with the insurance that they would be able to operate a profitable oil export industry when the civil war ends. The Safer vessel would be towed and sold for scrap. The Houthis would have no legal or commercial liability.

Gressly said the crisis was urgent since “in March, a UN-led mission to the Ras Isa peninsula confirmed that the 45-year-old supertanker is rapidly decaying. It is at imminent risk of spilling a massive amount of oil due to leakages or an explosion”. Inert air on the ship that normally inhibits an explosion has dissipated, the UN said.

Doug Weir at the Conflict and Environment Observatory said: “While some may question the $80m price tag of the UN-mediated plan to address the threat posed by the FSO Safer, the costs of inaction – which start at $20bn for managing the consequences of a catastrophic spill – are far, far greater. The world has watched this situation grow more dangerous with every passing month, and it’s vital that donors provide the money that is needed to allow this urgent plan to proceed this summer.”

Greenpeace said the UN plan must be implemented before October, “when the wind and the currents will be too dangerous and hinder any rescue operation. Lack of funding cannot be an excuse to fail.”

Constructed in 1976 as an oil tanker and converted a decade later to be a floating storage facility for oil, no structural repairs have been made to the 376-metre-long vessel since Yemen’s civil war started in 2015. It contains 36 different oil storage facilities amounting to a capacity of more than 3m barrels.

The unavailability of diesel fuel has meant that Safer’s engines have not been started for several years, and the structure has been exposed to humidity and corrosion with little or no maintenance.

A major spillage would force the closure of the ports of Hodeidah and As Salif – which are essential for commercial imports and the provision of life-saving humanitarian assistance. Depending on the season and prevailing wind and currents, the environmental impact could hit Saudi Arabia, Eritrea, Djibouti and Somalia, as well as disrupt shipping through the Bab al-Mandab Strait and the Red Sea.

The Red Sea remains highly biodiverse, with coral reefs, coastal mangroves and many endemic species.

The donor conference comes at a time of the first nationwide ceasefire in Yemen in six years and a Saudi-overseen change in the leadership of the Aden government. A new seven-strong anti-Houthi Presidential Leadership Council has been formed, a move that involves the long-serving and ailing President Abd Rabbu Mansour Hadi relinquishing his executive powers.