Those calling for pension funds to divest their fossil fuel holdings do not understand the “huge task” facing the schemes in divesting carbon-intensive companies, the head of one of the UK’s largest unions has argued.

Unison said efforts by campaign groups urging asset owners to divest fossil fuel holdings were “admirable”, but underestimated the cost and complexity of selling the stakes.

Dave Prentis, the union’s general secretary, said: “We all want to live in a greener, cleaner world, but pulling local government pensions fund investments from firms with big carbon footprints, and putting them into environmentally-friendly investments instead, is no mean feat.”

His comments came in response to NGOs including Friends of the Earth detailing the fossil fuel investments of local government pension schemes (LGPS) to coincide with a report that found over 400 asset owners worth $2.6trn (€2.3trn), ranging from foundations and pension schemes to sovereign wealth funds, had pledged to divest their holdings.

Prentis cautioned against a wholesale divestment before other assets were available.

“Divesting of carbon assets without having found adequate alternative renewable investment returns would create huge economic uncertainty.”

He argued that low-carbon investment opportunities in the UK remained limited and were often not of sufficient scale, with investors incurring “high fees and huge transaction costs”.

Prentis also evoked cost as a concern when arguing against divestment, insisting that any moves would take several years and see the affected LGPS incur “considerable” costs.

The Norwegian Government Pension Fund Global was ordered in May to sell its stake in companies that derived over 30% of their revenue from coal.

It came weeks after the sovereign wealth fund said it had already halved its exposure to thermal coal, and was the result of a vote by Norway’s parliament.

However, divestment has previously proven difficult for the UK’s LGPS, consisting of 101 schemes managed by local authorities, as legal advice instructed them they could only divest in cases where it did not risk “material financial detriment” to the schemes.

The Lothian Pension Fund in July ruled out divestment, citing both cost concerns and uncertainty on how fossil fuel companies should be defined.