LONDON - The chiefs of several investor-focused groups and firms with more than $1.5 trillion in combined assets have written to British Prime Minister Rishi Sunak to warn him against watering down the country's climate policies.

In a letter dated Sept. 28, the group urged the government to reconsider recently announced plans to delay the phase-out of new internal combustion engine car sales and gas boilers, and to stick to plans to toughen energy efficiency targets in homes.

The moves, as well as the green-lighting of fresh oil development in the North Sea, have drawn sharp criticism coming as they do just weeks ahead of the next round of global climate talks, where countries are being urged to cut emissions quicker.

"Diluting ambition at this critical juncture erodes the UK’s position as a global leader on climate, undermines our international competitiveness, and increases the risk that we fail to capitalise on one of the greatest economic opportunities of the 21st century," the investor group wrote.

Whilst announcing the plans last week, which will see combustion engine car sales banned in 2035 rather than 2030, Sunak said he wanted to ease the "unacceptable costs" on British households.

"We urge the government to uphold ambition and avoid backsliding on key climate policies.... In the absence of strong policy incentives from government, there is a significant risk that investment will flow to the regions and nations that are taking a more consistent, long-term approach," the investors wrote.

The letter was sent by the chief executives of the Institutional Investors Group on Climate Change, the Principals for Responsible Investment and the UK Sustainable Investment and Finance Association.

Among individual firms to lend their support to the letter were Aviva Investors, the fund arm of insurer Aviva, Jupiter Asset Management and Ninety One.

(Reporting by Simon Jessop; Editing by Hugh Lawson)