LONDON, Nov 4 (Reuters) - Global food and packaging companies, including Nestle NESN.S, PepsiCo PEP.O and Unilever ULVR.L have vowed to collaborate on cutting plastic use and push for regulation after U.N. talks in August collapsed, a report published on Tuesday showed.

The failure to reach a deal at U.N. talks in Geneva dimmed hopes of tackling a key source of pollution and left many advocates of restrictions pessimistic about securing a global deal while U.S. President Donald Trump is in office.

Corporate alliances seeking to limit the impact of global warming are under pressure as the Trump administration dismantles a range of climate protection initiatives, and though light on concrete targets, the 2030 Plastics Agenda for Business, published by the Ellen MacArthur Foundation, could give sustainability advocates some cause for optimism.

"It is certainly encouraging to see multinationals publicly recommit on plastics, but credibility now hinges on evidence, not new promises," Kelly Cooper, sustainability consultant and co-author of a Harvard study into the impact of political shifts on corporate sustainability, told Reuters.

Cooper and co-author Neil Hawkins, a consultant and research adviser at Harvard, wrote in the Harvard Business Review that corporate coalitions on climate change were weakening.

A majority of the 75 companies in their study were seeking to reduce political exposure and avoid activist backlash by limiting public comments on sustainability progress, a strategy known as greenhushing, they wrote.

Companies may be less keen to trumpet their sustainability work, Rob Opsomer, executive lead for plastics at the Ellen MacArthur Foundation, told Reuters, but the signatories to the global commitment, which account for around 20% of worldwide plastic packaging, tripled their use of recycled content between 2018 and 2024, far outpacing the global market.

The commitment, which calls for collaborative action between companies, is important for driving progress and showing policymakers over the next five years that solutions can be scaled and that the market is ready for effective regulation, Opsomer said.

"It's sometimes a bit of a chicken or egg situation," Opsomer said. "A policymaker needs to have confidence that they're not regulating on a pipe dream."

(Reporting by Alexander MarrowEditing by Mark Potter)

((alexander.marrow@thomsonreuters.com; +447393253522))