Whitehall departments have been given until Friday to disclose if senior officials have second jobs in the private sector or other potential conflicts of interest.
It follows the revelation that Whitehall’s former head of procurement was working for Greensill Capital while still a civil servant.
The demand emerged as the Treasury Select Committee said it would investigate the response of ministers – including the chancellor, Rishi Sunak – to lobbying by former prime minister David Cameron on behalf of Greensill.
Mr Cameron, who joined Greensill in a paid role as special adviser in 2018, two years after leaving Downing Street, was found to have sent text messages and emails to ministers in an effort to exert influence within government on behalf of the financial services company.
The new inquiry was announced less than an hour after Conservative MPs voted down Labour’s attempt to force a wider parliamentary probe.
An official investigation ordered by Mr Johnson earlier this week has been described by Labour as “wholly inadequate” and “an insult to us all”, amid questions over its independence.
Labour’s Rachel Reeves accused Mr Johnson’s party of voting “to cover up cronyism”.
“It’s the return of Tory sleaze: one rule for them, another for everybody else,” she said.
Earlier Mr Johnson’s spokesperson said the man chosen by the prime minister to head the official inquiry into the affair would not be paid for his other job in government for the duration.
Top lawyer Nigel Boardman is also a non-executive director at the business department.
No 10 said he would not act in that role while the inquiry was carried out and that he was not being paid “from now onwards”.
Mr Boardman, the son of a Tory former cabinet minister, has already come under fire after it emerged that his law firm previously campaigned against limited curbs to lobbying rules.
As discontent over the scandal grows among Tory MPs, Mr Johnson’s anti-corruption champion backed calls for a series of reforms on lobbying.
John Penrose, the Conservative MP for Weston-super-Mare, told MPs that there were rules designed to disclose who ministers meet.
“[But these] disclosures don’t happen fast enough, they aren’t complete enough, they aren’t mutually comprehensible and machine readable and searchable enough, and as a result it is much too difficult at the moment to link up who ministers have met with, who the lobbyists are working for, with who is donating money to which political party,” he said.
Mr Cameron has conceded it was a mistake to lobby ministers informally on behalf of Greensill, but has insisted he did not break any rules.
Fresh questions were raised last night as it emerged Mr Johnson had acted on a personal request from Saudi crown prince Mohammed Bin Salman who complained that his £300m deal to buy Newcastle United football club had been blocked.
The prime minister asked a senior aide to look into the Saudi complaint, it was reported — a move that could be seen as placing pressure on the Premier League.
Earlier Mr Johnson rejected Labour’s call for a wider inquiry into the lobbying scandal, claiming “it won’t do a blind bit of good”.
But he admitted it was unclear whether the “boundaries” that are supposed to exist between Whitehall and business had been “properly understood”.
He also told MPs that he did not recall the last time he had spoken to Mr Cameron.
“The honest truth is I cannot remember when I last spoke to Dave,” he said.
However, he insisted they had not discussed the Greensill affair.
The Treasury Select Committee inquiry will look at the “appropriateness” of the Treasury’s response to lobbying in relation to Greensill Capital.
Tory MP Mel Stride, the chair of the committee, said it would set out further details of the inquiry next week.
Conservative MPs voted down Labour’s proposal for a wider parliamentary inquiry by 357 votes to 262.
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