Labour and the Liberal Democrats have also called on the Conservatives to cease all attacks on the Electoral Commission and allow the body to conduct its probe “independently and without fear or favour”.
The commission triggered a political earthquake last week when it launched a formal probe into the recent refurbishment of Mr Johnson’s Downing Street flat, saying there were "reasonable grounds" to suspect offences may have been committed.
But the commission’s future looks uncertain after it was told last year to reform or face abolition by Amanda Milling, the Tory party co-chairman and a minister without portfolio in the cabinet office.
After this week’s probe was announced, Sir Iain Duncan Smith, the former Conservative leader, also criticised the body, saying: "For them to make this announcement – and the timing of it – does seem to me to be very questionable. Its reputation is in tatters over its very poor behaviour over the Vote Leave campaign.”
Cat Smith, Labour’s shadow minister for democracy, was accusatory of the Conservatives’ attempts to intimidate the Electoral Commission by saying they “come straight out of the Republican Party playbook”.
“This is just one part of a concerted strategy by the Tories to remove scrutiny and proper accountability,” she said.
“The Conservative government must make a clear statement revoking these threats.”
Ed Davey, the Liberal Democrat leader, has also written to Ms Milling, warning that while the commission’s investigation is underway “it would be entirely improper for you to continue to make threats regarding its future”.
He added that the watchdog “must not be given any reason to believe that their conclusions in this investigation could lead to repercussions for the commission from this government”.
The public will only have faith in the outcome of the probe if “you and your party immediately cease any attacks on the commission and allow them to operate independently and without fear or favour”.
The calls were backed by Sir Alistair Graham, the former chair of the Committee on Standards in Public Life.
He described the Tory chairwoman’s suggestion last August that the commission could be abolished as a “form of bullying”.
For the duration of the investigation, the Electoral Commission “should do their job as set out in the legislation without politicians interfering in any way. They are an independent body [and] they should be allowed to operate independently,” he said.
The Electoral Commission is seen as a particular bugbear of Brexiteers.
In 2018 the watchdog imposed a £61,000 fine on Vote Leave, the pro-Brexit campaign supported by Mr Johnson, which denied wrongdoing.
Tory MP and Brexiteer Peter Bone also accused the commission of being “arrogant, incompetent and vindictive”.
In the past, Electoral Commission insiders have been privately pessimistic they can hold off Conservative attempts to scrap or reform the watchdog.
A Conservative Party spokesperson said: “We believe all reportable donations have been transparently and correctly declared and published by the Electoral Commission.
“We will continue to work constructively with the Electoral Commission on this matter.
“While an investigation is ongoing we will not be commenting further.”
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