Some big oil companies say they want to play a key role in reducing carbon emissions, but they are set to be sidelined at the coming United Nations climate conference.

Retailers, tech companies and consumer products makers have all been named as “principal partners” for COP26 in Glasgow—allowing them greater involvement with the event—but major oil companies like BP PLC and Royal Dutch Shell PLC will have a lower profile. That is because the oil industry has yet to agree on science-based plans for how it will reduce carbon...

Some big oil companies say they want to play a key role in reducing carbon emissions, but they are set to be sidelined at the coming United Nations climate conference.

Retailers, tech companies and consumer products makers have all been named as “principal partners” for COP26 in Glasgow—allowing them greater involvement with the event—but major oil companies like BP PLC and Royal Dutch Shell PLC will have a lower profile. That is because the oil industry has yet to agree on science-based plans for how it will reduce carbon emissions, a hurdle U.K. organizers set as a prerequisite for involvement in the event.

Companies that haven’t set out credible emissions-reduction plans, as determined by a U.N.-backed third party, are unable to sponsor the event, run workshops and panel discussions, or book exhibition space in areas managed by the U.K. government.

“The COP26 Presidency is working most closely with organizations that are committed to taking real, positive action and have strong climate credentials,” a COP26 spokesman said. COP, which stands for conference of the parties, is the U.N.’s flagship annual climate event, and this year the U.K. is in charge of organizing it as the host country.

While the main focus of the meeting, which starts in Glasgow on Oct. 31, will be climate negotiations between governments, businesses have sought to engage with the event as they see both the risks and commercial opportunities of climate change.

BP and Shell officials are disappointed at being sidelined, people familiar with the matter said.

Photo: Christopher M. Matthews/The Wall Street Journal

Fossil-fuel companies have sometimes sponsored previous COP meetings, particularly when hosted in their home country, but their association drew criticism from environmental groups.

Officials at BP and Shell, which are both listed in London and have a large presence in the U.K., are disappointed at being sidelined at the event, people familiar with the matter said. Both companies had hoped to be involved, the people said, having set out plans to pivot away from oil and gas and invest more in low-carbon energy. The sidelining of the industry is seen as less important to U.S. rivals Exxon Mobil Corp. and Chevron Corp. because they have a smaller presence in Britain.

“COP26 is primarily for governments—we want it to be a success and stand ready to support the efforts of the U.K. government and other governments around the world in whatever way we can,” said a BP spokeswoman.

Oil companies still plan to send staff to COP and BP said Chief Executive Bernard Looney plans to attend. Company representatives can, as members of industry groups, attend the event as observers, which is how many business people go.

Shell said its “focus is on what can be achieved through the momentum leading up to COP26 and beyond.”

To participate in the event, the U.K. organizers required large companies to have carbon-reduction targets in line with the latest climate science on how much and how quickly they need to reduce their emissions. The Science Based Targets initiative, a partnership between the U.N. Global Compact and other nonprofit organizations, ultimately decides whether or not those plans are credible. Alternatively, companies could sign up for the U.N.-backed Race to Zero campaign, which requires science-based emissions-reduction targets.

Companies voluntarily work with the SBTi to form an emissions-reduction plan for their industry. Several sectors, including aviation and financial services, already have targets accepted.

Plans for the oil-and-gas industry are still in development, having been delayed partly because of their complexity, a spokeswoman for the SBTi said.

“Strenuous efforts have been made on both sides but haven’t succeeded,” a person familiar with the matter said.

Other energy companies, like utilities SSE PLC and ScottishPower, which is owned by Iberdrola SA, are sponsoring COP26 after their emissions-reduction plans were ratified by the SBTi. Corporate sponsors also include Microsoft Corp. and Dove soap maker Unilever PLC.

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“I’ve not heard of any COP previously restricting who can participate by requiring a science-based commitment,” said Dirk Forrister, chief executive of the International Emissions Trading Association, of which BP and Shell are members. “I think the U.K. government is definitely trying to send a message that the fossil fuel industry is under the gun to lead the energy transition and set targets for how they will do that.”

Several European oil-and-gas companies, including BP and Shell, have set goals to reach net zero carbon emissions by 2050, although activists, investors and governments continue to pressure the industry to move faster and give more detail about their plans.

“It’s imperative that the Glasgow meeting calls for the highest levels of ambition in terms of immediate emissions reductions,” said Nigel Topping, the U.K.’s high-level climate action champion, responsible for driving action from businesses, investors, organizations and regions on climate change. Existing commitments from the oil industry are insufficient and don’t align with global climate goals, he added.

“It [COP] cannot offer a platform to entities that do not meet this level of commitment,” Mr. Topping said.

Write to Sarah McFarlane at sarah.mcfarlane@wsj.com