Angela Rayner has accused the Tories under Liz Truss of being "devoid of ethics" amid questions over her appointment of the No 10 chief of staff.
It comes after The Guardian reported the Prime Minister had promised Mark Fullbrook his company would run the Tories' next election campaign if he joined her No 10 team.
A No 10 source declined to comment on the claim and the PM's official spokesperson said: "Decisions on running electoral campaigns of any party would not be one for me".
Mr Fullbrook's represenatatives have been contacted for comment regarding the claim.
But Labour's deputy leader said the Conservatives are "already sowing the seeds of a dirty general election campaign".
She added: "This is not only an inappropriate appointment at the heart of Downing Street but raises serious questions about what sensitive and privileged information this on-loan lobbyist will have access to at the heart of government.
"The Tories under Liz Truss are devoid of ethics. A Labour government will clean up politics with an Independent Ethics and Integrity Commission to restore standards in public life".
Over the weekend it also emerged Mr Fullbrook is having his salary paid via his lobbying firm rather directly as government staff.
Instead his wages are paid by Fullbrook Solutions, a lobbying firm he set up in April but which has since suspended commercial activity.
The Cabinet Office claimed it is "not unusual" for a special adviser to join government "on secondment".
A spokesperson said: "All government employees are subject to the necessary checks and vetting, and all special advisers declare their interests in line with Cabinet Office guidance.
"It is not unusual for a special adviser or civil servant to join government on secondment. Any government employee hired on secondment is subject to the usual special adviser or civil service codes.
"The government will pay the salary of an employee on secondment, including costs such as Employers National Insurance contributions to the seconding company. This has been cleared by the Propriety and Ethics team in Cabinet Office."
A spokesperson for Mr Fullbrook added: "This is not an unusual arrangement. It was not put in place for tax purposes and Mr Fullbrook derives no tax benefit from it."
