A senior civil servant worked for Greensill Capital – the financial firm at the centre of the David Cameron lobbying row – while still employed in Whitehall, it has been revealed.

Questions have now been asked about why Bill Crothers did not consult a Whitehall watchdog before joining Greensill as a director.

He began working for the company in an advisory role in September 2015 and did not leave his job as government chief commercial officer at the Cabinet Office, in charge of billions of pounds worth of government contracts, until November that year.

Less than 10 months later, he became director of the recently-collapsed finance firm.

The latest revelation came following correspondence between the Advisory Committee on Business Appointments (ACOBA) and the Cabinet Office.

Lord Pickles, chair of ACOBA, has today written to the head of the civil service, Alex Chisholm, to ask why Mr Crothers did not seek advice about joining Greensill.

According to the Cabinet Office, Mr Crothers did not need to consult ACOBA on his appointment at Greensill once he had left the civil service – as is usual for former ministers and top officials taking on private-sector roles – because he “was already working in an advisory capacity to Greensill before he left the civil service”.

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The Cabinet Office had allowed him to advise Greensill part-time for his final three months at the civil service.

Mr Pickles wrote: “The lack of transparency around this part-time employment with Greensill may have left the misleading impression that Mr Crothers had wilfully ignored the obligation to seek advice.”

The revelation is likely to prompt a fresh round of questions about the links between Greensill Capital and senior ministers and officials, which Prime Minister Boris Johnson has already ordered a top lawyer to investigate.

Over the past few weeks, Greensill Capital has come under scrutiny over allegations about links between Australian financier Lex Greensill, who owns the firm, former prime minister Mr Cameron and government ministers and officials.

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Cameron breaks silence over Greensill

Mr Cameron, who was employed by Mr Greensill in 2018 after he left Downing Street, has been revealed to have approached serving ministers about the involvement of Greensill Capital in government-backed financial support schemes during the coronavirus crisis.

This included text messages sent to Chancellor Rishi Sunak.

Rachel Reeves, shadow chancellor of the Duchy of Lancaster, said the fact Mr Crother was allowed to work as an adviser for Greensill while a civil servant is “an extraordinary and shocking revelation”.

“The Conservatives have weakened the rules so much they may as well rip them up and start again,” she said.

“They must be kidding themselves if they think the current checks and measures they’ve got in place are working.

“They need to strengthen rules now and get everything about the Greensill scandal out in the open with a proper parliamentary inquiry.”