A fundraising event next month will attempt to secure the final $29m needed to remove 1.14m barrels of oil from a decaying tanker off the coast of Yemen.

The UN appeal has bought a VLCC to receive oil from the 406,600-dwt floating “time-bomb” FSO Safer (built 1976) but the spiralling cost of the operation means more money is needed.

The conference, co-hosted by the UK and Dutch governments, will attempt to fill the funding gap and allow the salvage operation to start. The appeal has nearly hit $100m, United Nations officials said.

The 307,000-dwt VLCC Nautica (built 2008) has been converted into an FSO at a yard in China and the vessel is currently off the coast of Singapore en route to Yemen, according to Kpler tracking data.

The UN bought the Nautica from Belgian tanker giant Euronav but rising asset values pushed the price up to $55m — a sum not covered in the original budget.

“The replacement vessel has begun its journey to Yemen but there is not enough funding for the salvage operation to take place,” ambassador Barbara Wood, the UK’s permanent representative at the UN, said.

“The costs of inaction are severe. This would devastate marine life and coastal livelihoods, disrupt live-saving humanitarian assistance for 17m people and cost the global economy billions in lost trade every day.”

The conference will be held on 4 May for the final funding push for the salvage operation, the UK government said.

“This event aims to fill the shortfall and provide a long-term solution for Yemen,” Woodward said. “It is on all of us, states, private sector, individuals, to step up and help. The time to act is now.”

The long-delayed operation followed sluggish efforts to raise the money despite the threat of the ship breaking up leading to a spill in the Red Sea that would be four times the size of the Exxon Valdez disaster.

“[The FSO] Safer could break apart or explode at any time, unleashing an environmental, trade and humanitarian catastrophe,” UK development minister Andrew Mitchell warned.

The Nautica is expected to arrive at the FSO Safer off Yemen’s Red Sea coast in May.

The Nautica will remain there for months after receiving the FSO Safer’s oil and Euronav will help operate its former ship throughout that period.

“The UN thanks member states, the private sector and general public for providing $99.6m to prevent a massive oil spill in the Red Sea from the FSO Safer,” David Gressley, the UN’s resident humanitarian coordinator in Yemen, wrote on Twitter.

“We’re closer than ever to averting a catastrophe, but urgently need $29m more to start the salvage operation.”