A group of former Pacific leaders want the UN agency mandated to regulate international shipping "to take meaningful strides" to end the industry's reliance on dirty fossil fuels.

The International Maritime Organisation (IMO) is meeting for a two-part climate summit in London over the next two weeks with the aim to arrive at a key agreement on cutting pollution from shipping.

The Pacific Elders Voice group said the issues on the negotiating table at the 15th Intersessional Working Group on Reduction of Greenhouse Gas Emissions from Ships (ISWG-GHG-15) from June 26-30 and the 80th Marine Environment Protection Committee (MEPC80) meeting from July 3-5 "will have long-term consequences for our economic and societal futures".

The leaders - which include former president of the Marshall Islands Hilda Heine, former president of Palau Tommy Remengasau, former president of Kiribati Anote Tong, and the former prime minister of Tuvalu Enele Sopoaga as well as former secretary-general of the Pacific Islands Forum Dame Meg Taylor - said shipping is a lifeline for the Pacific, providing "access to crucial energy and supplies needed to power our economies".

"However, shipping is also a significant source of historical and contemporary emissions that have also contributed to climate harms in the past and will continue to harm the climate system," the group said.

"The climate harms caused by shipping are the negative externalities that have yet to be mitigated and compensated by the industry."

The group sees the MEPC80 as the "last opportunity" for the sector - responsible for 90 percent of global trade - to fully decarbonise by 2050.

"We would also like to highlight the significance of this meeting for shipping, regional and international trade and climate action generally. The matters under negotiation at MEPC80 will have long-term consequences for our economic and societal futures," they said.

"As many commentators and experts have noted, MEPC80 is the industry's last opportunity to secure a 1.5 degrees-aligned pathway." The former leaders also commended the alliance of Pacific nations, often referred to as 6PAC, who have been actively campaigning at the IMO for 1.5 degrees and for an equitable transition from fossil fuels to be embedded in the IMO's Revised Strategy.