Fossil fuel firms have been given no official role in the Cop26 climate summit, it can be revealed, against a background of growing concern among UK officials that big oil’s net zero plans do not stack up.

Private emails from civil servants in the Cop unit, seen by the Guardian, show doubts about one oil major’s net zero plans, with an official saying BP “[does] not currently fit our success criteria for Cop26” and another noting “it’s unclear whether [its net zero] commitments stack up yet”.

Last year the Guardian revealed that fossil fuel firms had held a series of private meetings with UK officials in an attempt to be part of Cop26. Documents revealed that some of the world’s biggest polluters had been lobbying the government, offering money in return for exposure at the event and in one case saying they could act as an intermediary between UK officials and other governments.

But now, in what campaigners say is a big win for climate activism, the UK’s Cop unit has confirmed that no fossil fuel majors will have a formal role.

Chris Garrard, of the campaign group Culture Unstained, which obtained the emails under freedom of information legislation, said: “For years oil companies have been given prominent platforms at the UN climate negotiations, promoting themselves as climate leaders while they continued to pour millions into new fossil fuels, so this is a big step forward.”

The UK government has been widely criticised before Cop26 for its continued support for fossil fuel extraction – from new oilfield exploration in the North Sea to a multimillion-pound investment in a gas terminal in Africa.

Garrard said the fact that oil majors would have no formal role at Cop26 showed that “the organisers have conceded that the oil industry’s claims of going net zero don’t add up” and that the firms’ business plans “directly undermine” global climate targets.

Last month the UK’s Cop26 climate champion, Nigel Topping, who is responsible for driving action from businesses and investors, underlined the government’s hardening stance toward fossil fuel corporations, saying current commitments from the oil industry were insufficient and did not align with global climate goals.

“It’s imperative that the Glasgow meeting calls for the highest levels of ambition in terms of immediate emissions reductions,” Topping told the Wall Street Journal. “It [Cop26] cannot offer a platform to entities that do not meet this level of commitment.”

A Cop26 spokesperson said officials from big oil may still attend fringe events but confirmed they would have no formal role.

In a statement, they said: “[We] are working most closely with organisations that are committed to taking real, positive action and have strong climate credentials. Companies who have committed to achieving net zero, and have published a plan of action of how they will do this, such as through UN-backed science-based targets.”

Fossil fuel companies have sponsored some events at previous Cop meetings, drawing sharp criticism from environmental groups. Campaigners say the decision to exclude them this time is a sign that the corporations’ “social licence is being withdrawn”. It follows successful campaigns across the arts and higher education to end sponsorship deals and investments.

Garrard said: “Thanks to the anti-oil sponsorship and divestment movements, we have seen a dramatic shift in public opinion and fossil fuel companies are increasingly seen as unacceptable partners.”

The emails, from September last year, show discussions between civil servants in the Cop unit and the British embassy in Poland before a Cop26 meeting that was due to feature a representative from BP.

They appear to reveal concern in the Cop unit about the company’s net zero plans, with one official saying BP did “not currently fit our success criteria for Cop26, so any speaking attendance would not be appropriate” and “we strongly recommend not to use Cop26 branding on the comms around your event if BP are speaking”. Another civil servant noted that “it’s unclear whether BP’s [net zero] commitments stack up yet”.

The meeting did go ahead but all Cop26 branding was removed.

BP’s plan to reach net zero by 2050, unveiled last year, has been criticised by campaigners and climate scientists.

A BP spokesperson told the Guardian that as well as its 2050 target it had set nearer-term goals for emissions reductions for 2025 and 2030.

“Our ambition is to be a net zero company by 2050 or sooner and to help the world get to net zero … We believe that our net zero ambition and aims are, collectively, consistent with the Paris goals,” they added.