The Financial Reporting Council (FRC) should hold firms signed up to a sustainable investment code to higher standards, campaigners have said.

The reporting watchdog announced that a record number of companies are signed up to the UK Stewardship Code, which sets high sustainability standards for those investing money on behalf of UK savers and pensioners.

But ShareAction, which campaigns for responsible investment, said there is a lack of transparency in how companies are added, re-added or removed from the code.

It comes as NatWest Group Pension Scheme and JO Hambro Capital Management are no longer signatories while 27 organisations were added and 164 successfully renewed their status.

Asset owners, asset managers and service providers can apply or reapply to become signatories by submitting stewardship reports to the FRC.

But signatories can also be delisted due to the quality of their reporting, if they fail to take the FRC’s feedback into account or if they choose not to reapply.

ShareAction warned that the latest round of applications to join the Stewardship Code “demonstrates a concerning lack of rigour in holding existing signatories to account for their commitments”.

Lewis Johnston, director of policy, said: “The Stewardship Code is rightly respected around the world and has been effective in driving up standards for responsible investment in recent years.

“However, as recent public announcements and our own ShareAction research makes clear, the stewardship performance of some code signatories has been questionable at best and is difficult to reconcile with some of the code principles.

Some signatories such as Vanguard, Vontobel Asset Management and Manulife Investment Management had a low ranking in the campaign group’s report of the world’s 77 largest asset managers’ approaches to responsible investment.

Mr Johnston said the process for making decisions on whether a signatory should be delisted is “opaque”, adding that the FRC’s rationale for readmission and delisting can seem “unclear and arbitrary”.

“We would like the FRC to reaffirm the high standards expected of code signatories, and to specify the grounds on which members may be delisted,” he said.

“This will ensure that the code can maintain its deserved reputation as a pioneering initiative for responsible investment and ensure that it can continue to promote the highest possible standards for stewardship amongst institutional investors making investment decisions on behalf of millions of people in the UK and beyond.”

Among the new signatories are Phoenix Group, Canada Life, Aegeon, ISS, Ardea Asset Management and Atlas Infrastructure.

The FRC said it is pleased to see continued growth in the assets other than listed equity covered by the code.

It also welcomed signatories’ progress in sharing their stewardship activities and outcomes, such as improving board diversity at investee companies and improving disclosure related to climate change and biodiversity.

The PA news agency has contacted the FRC for comment.