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 a powerful Commons 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.
The new probe was announced less than an hour after Conservative MPs voted down Labour’s attempt to force a wider parliamentary inquiry.
An official investigation ordered by Boris 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 spokesman said the man chosen by the prime minister to head the official probe 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 former Tory Cabinet minister, has already come under fire after it emerged 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 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.
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 that the “boundaries” that are supposed to exist between Whitehall and business had been “properly understood”.
He also told MPs did not recall the last time he spoke 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 committee inquiry will focus 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 next week.
Conservative MPs voted down Labour’s proposal for a wider parliamentary inquiry by 357 votes to 262.