Gibson Dunn & Crutcher has stopped lobbying on behalf of Saudi Arabia against US legislation affecting oil-producing countries, becoming the latest firm to end ties with the kingdom amid international outcry over the alleged killing of dissident Saudi journalist Jamal Khashoggi.

Three Gibson Dunn lawyers – including high-profile litigator Ted Olson – were named in August on a $250,000 flat-fee lobbying contract for the kingdom, and the advocacy included the preparation of a whitepaper opposing the passage of the No Oil Producing and Exporting Cartels Act, widely known as NOPEC.

The firm also agreed to prepare an op-ed challenging the legislation, along with an in-depth analysis of the bill, introduced this year in the US House of Representatives.

The firm has declined to expand the scope of its lobbying to include meetings between Olson – a former US solicitor general under the George W Bush administration – and members of Congress. Its agreement with Saudi Arabia included an option to include that advocacy for a monthly fee of $100,000. The end of the representation was revealed in a regulatory filing obtained by Legal Week sister title the National Law Journal.

The move comes on the heels of Glover Park Group and BGR Group deciding to cut ties with Saudi Arabia as the kingdom comes under increasing scrutiny over the disappearance of Khashoggi, who went missing more than two weeks ago after visiting the Saudi consulate in Istanbul. On Thursday, Treasury Secretary Steven Mnuchin said he would no longer participate in an upcoming investment conference in Saudi Arabia. Other business and tech executives also have dropped out of the Future Investment Initiative in Riyadh.

The kingdom has denied any role in Khashoggi's disappearance. A person who answered the phone at the Saudi embassy in Washington on Thursday said no one was available to provide a comment.

Gibson Dunn declined to comment on the conclusion of its contract, a client relationship that required the firm to file papers with the US Justice Department under the Foreign Agents Registrations Act, or FARA, which requires the disclosure of US lobbying activity for foreign governments. In addition to Olson, Gibson Dunn partner Amir Tayrani and associate Benjamin Olson registered to represent Saudi Arabia.

Gibson Dunn's registration to represent Saudi Arabia, dated 7 September, marked its first filing under FARA in more than 20 years. The firm had previously represented Saudi Arabia in the mid-1990s.

Gibson Dunn's contract included broad language outlining scenarios that would allow it to drop the kingdom as a client. The agreement gave Gibson Dunn "the right to withdraw from this representation" in the event that Saudi Arabia failed to pay the firm in a "timely manner" or if it failed to follow its advice on a "material matter". Another scenario envisioned in the contract terms was more open to interpretation: the emergence of a "fact or circumstance" that would, in Gibson Dunn's view, "render our continuing representation unlawful or unethical".

Several other US law firms filed disclosures under FARA this year in connection with their lobbying work for Saudi Arabia, including Hogan Lovells, which is paid a monthly retainer of $125,000 to provide "strategic and legal advice on legislative, regulatory and public policy activities of interest".

Among the Hogan Lovells lawyers advising Saudi Arabia is former US Senator Norm Coleman, now senior counsel at the firm. The agreement is set to expire at the end of the year. A Hogan Lovells spokesman declined to comment.