"Any attempt to bend the listing rules in order to facilitate the IPO of Saudi Aramco is highly inappropriate and flagrantly ignores the principles which the UK’s listing rules were designed to defend," Ashley Hamilton Claxton, corporate governance manager at Royal London, said in a statement.

Exchanges around the world are vying for a piece of Saudi Aramco's IPO, which is expected to be the largest in history.

The London Stock Exchange and the British regulator are working on a new model that would allow the firm to avoid the most onerous corporate governance requirements of a primary listing, without being seen as second class.

"We will be lobbying strongly against any concessions being granted should there be a formal attempt to IPO Aramco in the UK," Hamilton Claxton said.

"As long-term investors in the UK equity market we fear this precedent could lead to a slippery slope."

The Financial Conduct Authority, the UK's financial watchdog, declined to comment.

(Reporting by Carolyn Cohn; editing by Dasha Afanasieva and Jason Neely)