Companies that stumped up millions of pounds to sponsor the Cop26 climate summit have condemned it as “mismanaged” and “very last minute” in a volley of complaints as next month’s event in Glasgow draws near.

The sponsors, which include some of Britain’s biggest companies, have raised formal complaints blaming “very inexperienced” civil servants for delayed decisions, poor communication and a breakdown in relations between the organisers and firms in the run-up to the landmark talks.

The Guardian understands that a letter to the organisers, written by broadcaster Sky and co-signed by senior leaders from other Cop26 sponsors, has raised concerns with them over these and other problems, and followed another co-signed letter in July.

The UK is running its Cop26 presidency from within the Cabinet Office, under the leadership of the former business secretary Alok Sharma, who is the Cop26 president, and the businessman Nigel Topping who was appointed the government’s high-level climate action champion last year. Sponsorship is expected to help defray a policing bill estimated to reach up to GBP250m.

Alongside Sky, the summit has 10 other major sponsors, including energy giants Hitachi, National Grid, Scottish Power and SSE, US tech titan Microsoft, and FTSE companies GSK, NatWest, Reckitt, Sainsbury’s and Unilever. Unilever has denied signing the letter penned by Sky. Other lower tier “partners” include the car maker Jaguar Land Rover and the furniture retailer Ikea.

One source, employed by a Cop26 sponsor, said that “the biggest frustration” was the lack of information about how the event will run, and the role for its key backers, because important questions have gone unanswered and planning decisions have been delayed.

“They had an extra year to prepare for Cop due to Covid, but it doesn’t feel like this time was used to make better progress. Everything feels very last minute,” the source said.

The upcoming climate talks, considered the last chance to put the world on track to meet its climate ambitions, are due to take place in early November after the event was postponed by a year because of the outbreak of Covid-19 in early 2020.

They have already been thrown into turmoil by suggestions that the Chinese president, Xi Jinping, will skip the event, threatening the chances of a global pact with the world’s biggest carbon dioxide emitter.

Organisers of Cop26 promised sponsors an “outstanding opportunity” and “unique benefits” in exchange for their support, including a chance to promote their brands at the conference “green zone” exhibition space and the participation of government ministers at their events.

But in multiple emails and official letters the companies have complained to organisers about unmet expectations, and deepening concerns over delays to the green zone plans. They have also raised complaints that ministers have not always been available for their events in the run-up to Cop26, as agreed as part of the sponsor deals.

Other sources have described the “shifting goal posts” and “inertia” plaguing the Cop26 planning as “deeply frustrating”.

Many of the event’s corporate backers regularly take part in high-profile sponsorship deals for big events, and have been left bewildered by the slow progress of the Cop26 events, another source explained.

The source blamed the “very young, very inexperienced” civil servants tasked with planning the event for taking a “top-down public sector approach” that has raised hackles among sponsors.

“It’s clear that many of them have very little experience managing relationships in the private sector, or even experience attending a Cop event,” the source said.

The energy company sponsors – Hitachi, National Grid, Scottish Power and SSE – are understood to be particularly frustrated because they were under the impression that no other energy brands would feature at Cop26. However, the “blue zone”, which is organised by the UN, will include rival brands.

Ministers had been due to release three key documents on Monday on the government’s plans to achieve its net zero target by 2035, but publication has been delayed owing to the murder of the MP Sir David Amess.

The documents reveal a stark split within the cabinet, understood to be between on one side Boris Johnson, the prime minister, Kwasi Kwarteng, the business secretary, and Michael Gove, responsible for improving the UK’s homes, all seeing benefits to strong climate action; and on the other, the chancellor, Rishi Sunak, a free-market hawk instinctively opposed to government intervention.

All three papers are now expected to come later in the week, with the government seeking to preserve a show of unity over the publications.

The row over the government’s handling of Cop26 planning has emerged amid public order concerns, with up to 150,000 protesters expected to take to Glasgow’s streets in early November alongside the crucial climate talks, which will require one of the largest policing operations ever undertaken in Britain.

Countries and organisations planning to host events have also said they fear that increased costs will cause problems for developing nations.

Multiple participants told the Guardian earlier this month that the cost of renting Cop26 pavilions – event spaces for hosting workshops, panel discussions and keynote speeches during the conference – is considerably higher than it was at Cop25 in Madrid, with some saying it had increased by as much as 30%.

A Cop26 spokesperson said the organisers were “working closely” with sponsors which would increase the value-for-money for taxpayers, and reduce the overall financial cost of Cop26.

A Whitehall veteran of Cop summits said: “It feels like some of these sponsors have forgotten the actual reason we’re in Glasgow. Cop isn’t about branding, it’s about tackling climate change. Keeping 1.5C in reach is the best thing you can do for your bottom line: they would do well to remember this.”

Additional reporting Fiona Harvey