By Simon Jessop, Virginia Furness and Kate Abnett

LONDON (Reuters) -A group of development banks plans to invest at least 3 billion euros ($3.4 billion) by the end of the decade in tackling plastic pollution in the sea, expanding the scope and financial firepower of what remains the world's biggest effort to fix the growing problem.

The U.N. estimates that, on current trends, plastic waste entering the water could triple to up to 37 million metric tons per year by 2040, from around 11 million tons in 2021.

Of particular concern is the growth in microplastics, which have contaminated all the major oceans, as well as the soil and air, finding their way into animals, plants and humans.

Launching the second iteration of the Clean Oceans Initiative (COI) as a U.N. conference kicks off in Nice, France, the European Investment Bank's project lead, Stefanie Lindenberg, said the amount could rise further as other partners join.

Also including French, German, Spanish and Italian lenders and the European Bank for Reconstruction and Development, the original initiative deployed 4 billion euros of pledged investments between 2018 and May 2025, ahead of a year-end target, said EIB Vice President Ambroise Fayolle.

With a focus on better managing solid waste, wastewater and stormwater, projects in the initial phase included improving wastewater treatment in Sri Lanka, solid waste management in Togo and flood protection in Benin.

While that will remain a focus for the next phase of investment, the project will expand to target upstream sources of waste, for example by helping develop new forms of packaging and ensuring more waste is recycled.

"We see that there is a role for us," Lindenberg said. "To reduce the amount of virgin plastic that is needed or, at least, can be kept in the system."

The bank could help lower the risk of developing new technologies, types of packaging and products, for example by providing cheaper funding, grants or investment into third-party funds, she said.

As well as helping build out a pipeline of investable projects, the programme would look to work closely with development banks in other regions, particularly those operating in Asia and Latin America, both leading sources of ocean waste.

Alongside the Asian Development Bank, which has already joined the second phase and is expected to provide strong local knowledge and contacts, talks are ongoing with the World Bank and Inter-American Development Bank, she said.

Countries will convene in August to try and hammer out a deal to curb plastic pollution, after they failed to agree one at December talks in Busan, South Korea.

($1 = 0.8754 euros)

(Editing by Kirsten Donovan)