The government has defeated Labour plans for a parliamentary inquiry into lobbying, amid a row over contacts with finance firm Greensill Capital.
Sir Keir Starmer had wanted a "full" probe, including public hearings by a cross-party panel of MPs.
Boris Johnson has insisted the lawyer he appointed this week to carry out a review of the affair will lead a "proper" inquiry.
And he said the Tories had been "consistently tough on lobbying".
Earlier, Sir Keir said the Greensill affair demonstrated "sleaze and cronyism" at the "heart" of the Conservative Party.
A row about lobbying has centred on attempts by Greensill Capital, a specialist bank which collapsed in March, to influence government ministers and top civil servants.
On Wednesday, the Commons Treasury select committee said it planned to launch an inquiry into the "lessons" from the firm's demise.
The government has appointed lawyer Nigel Boardman to look into the matter and report back by the end of June.
Labour said a cross-party Commons committee should investigate instead, with a wider remit to recommend changes to lobbying rules.
But speaking earlier at Prime Minister's Questions, Mr Johnson said Labour's proposals would involve MPs "marking their own homework".
He ordered Tory MPs to vote against the plan, which was defeated by 357 votes to 262.
Sleaze - amid stiff competition, it's perhaps the dirtiest word in politics.
It's used as shorthand for behaviour that's immoral, inappropriate, corrupt or just plain wrong.
And Labour's choice to attach it to the current government's actions is no accident.
Sir Keir Starmer is seeking to evoke the scandals of the past which have led to the downfall of Conservative ministers.
In the latter years of John Major's government in the 1990s accusations of sleaze were so commonplace it was arguably a factor in them losing power.
But while Boris Johnson and his ministers will be keen to avoid accusations of improper behaviour in office, there are dangers for the opposition here too.
Labour governments have seen their fair share of scandal in the past and it's a brave politician of any colour who claims to be beyond reproach.
Calls to examine lobbying and the relationship between civil servants, politicians and and private firms have intensified in the past week.
These have followed revelations about former Prime Minister David Cameron's efforts to influence ministers on behalf of Greensill, where he started working as an adviser in August 2018, two years after he left Downing Street.
These involved texting Chancellor Rishi Sunak and contacting other ministers about a government-backed Covid loan scheme on behalf of the company.
Along with the company's founder Lex Greensill, Mr Cameron also met Health Secretary Matt Hancock for a "private drink" in 2019 to discuss a new payment scheme for NHS staff.
What is lobbying?
- It's another word for trying to persuade the government to change its policies
- It can be done by individuals, companies, organisations and charities who contact ministers, backbench MPs and other politicians
- Some organisations and companies employ professional lobbyists to make their case for them
- Former MPs and civil servants often work as lobbyists
Speaking earlier, Sir Keir suggested there was a "revolving door" between the government and the private sector.
He told MPs: "I know the prime minister is launching an inquiry. That inquiry isn't even looking at the lobbying rules.
"I'm not sure it's looking at very much at all, because every day there's further evidence of the sleaze that's now at the heart of this Conservative government."
But the prime minster said Mr Boardman would be leading a "proper independent review," and Labour's committee idea would "not do a blind bit of good".
Mr Cameron has insisted he did not break any codes of conduct or rules on lobbying, although he has acknowledged that he should have communicated with the government "through only the most formal of channels".
Conservative MP William Wragg, who chairs a Commons committee currently looking into business appointments, said the Greensill affair was a "tasteless, slap-dash and unbecoming episode for any former prime minister".
But, speaking ahead of Wednesday's vote, he suggested Mr Cameron's involvement was a "red herring" compared with the wider issues - such as why companies were interested in employing "de-skilled" former ministers at all.
Mr Greensill, who worked as an unpaid government adviser during Mr Cameron's early years in Downing Street, has not commented on the row.
Mr Cameron has said the idea of him working at Greensill Capital "was never raised, or considered by me, until well after I left office".