Voters are shifting support away from the Tories, while two-thirds consider the governing party "very sleazy" amid a flurry of negative headlines, according to new polling.
A poll by Savanta ComRes for the Daily Mail put Labour six points ahead of the Conservatives in a sign that recent revelations about MPs' second jobs and the handling of the Owen Paterson affair could hurt Boris Johnson' s party at the ballot box.
The results, in which the Tories dropped four points and Labour climbed five since a similar poll a week ago, are based on a survey of more than 2,000 people in the UK, conducted on Thursday and Friday.
YouGov polling published on Friday and carried out this week also suggested the Tories had lost their lead, with its findings putting Mr Johnson and Sir Keir Starmer's Labour Party neck and neck on 35% of the vote share.
The polling, published in The Times, found two-thirds of voters viewed the Tories as "very sleazy" following Mr Paterson's resignation and MP second jobs and homes revelations.
A separate survey by Redfield & Wilton Strategies on Wednesday put Labour two points ahead of the Tories.
The findings come after the Government attempted to rip up the current Commons standards system to delay former Tory cabinet minister Mr Paterson's suspension for breaking lobbying rules, and revelations former attorney general Sir Geoffrey Cox voted by proxy while offering legal services in the Caribbean.
Speaking before the Savanta ComRes' results were published, the Prime Minister had rejected suggestions the sleaze allegations being levelled at his party could be reflected at the ballot box in forthcoming by-elections.
Mr Johnson visited Sidcup on Friday afternoon, where voters will elect a new MP in December after the death of former security minister James Brokenshire last month.
The Prime Minister said he did not "underestimate the vital importance" of MPs refraining from engaging in paid advocacy.
A Tory MP from the 2019 in-take said he thought the sleaze row, which he admitted had divided his party, had provided Labour with an opportunity to gain ground.
James Sunderland, MP for Bracknell, told the PA news agency: "The left are not in government, they want to be in government, and they're looking for weakness.
"And I think in many ways, what has happened over the last week or so has provided that opportunity for them."
The backbencher said the Tories needed to "restore" their "reputational integrity" in the face of criticisms of the Government's handling of the situation.
The comments and polling come as Lord Evans, chairman of the Committee on Standards in Public Life, said the public cared about issues such as standards - a point recent polling surveys appear to bear out.
He said: "The past week has shown that standards do matter to the public. Ethical standards are important for making democracy work. The public does care about this.”
The Times reported 14 MPs were taking advantage of a loophole in the Parliamentary expenses scheme which means they can let their homes to tenants, and then claim for rent paid on a London rental property to live in.