Boris Johnson and Carrie Symonds may have to hand over personal phone and email messages to the inquiry into how the refurbishment of their Downing Street flat was funded.
The Electoral Commission, who launched a formal investigation yesterday, also has powers to demand documents and information and could interview witnesses under caution - including the Prime Minister.
The watchdog said there are “reasonable grounds” to suspect an offence may have occurred.
Mr Johnson has insisted he has not broken any laws over renovations of his Number 11 residence and Downing Street said he would be “happy” to assist the commission.
However, The Times - quoting a Government source - has reported that Downing Street is concerned “there could be a paper trail”.
Investigation ‘could be over in five minutes’
Labour leader Sir Keir Starmer said an investigation into the flat renovations could be “over in five minutes” if he revealed “who paid for it in the first place”.
Sir Keir told ITV’s Peston: “He’s got the Cabinet Secretary doing an investigation, got the Electoral Commission doing an investigation.
“Those investigations could be over in five minutes if the Prime Minister just answered the question, who paid for it in the first place?”
Challenged by the Labour leader over the issue at Prime Minister’s Questions on Wednesday, Mr Johnson said he “personally” paid for the renovations, but refused to answer whether he received an initial loan from the Tory party.
Questions have been mounting over the flat since former aide Dominic Cummings accused Mr Johnson of wanting donors to “secretly pay” for the renovations to the apartment in a “possibly illegal” move.
What the Electoral Commission said
The Electoral Commission said the investigation will “determine whether any transactions relating” to the renovations “fall within the regime regulated by the commission and whether such funding was reported as required”.
The Conservative Party said it would “continue to work constructively” with the commission.
“We believe all reportable donations have been transparently and correctly declared and published by the Electoral Commission,” a spokesman said.
How much Prime Ministers get for flat renovations
Prime ministers get a budget of up to £30,000 per year to renovate their Downing Street residency, but newspaper reports have suggested Mr Johnson has spent up to £200,000.
Last week, the Daily Mail published details of an email from Tory peer Lord Brownlow in which he said he was making a £58,000 donation to the party “to cover the payments the party has already made on behalf of the soon-to-be-formed ‘Downing Street Trust’”.
Labour peer Baroness Jay told BBC Newsnight on Wednesday that she had been approached earlier this year by Lord Brownlow, who “asked if I would like to become a trustee on the Downing Street Trust”.
She said that the proposed trust “would be for the public part of Downing Street not the residences”, although nothing formal was ever agreed.
PM ‘ultimate arbiter’ of whether code has been broken
Former private secretary to the Queen Lord Geidt was appointed as the new independent adviser on ministers’ interests on Wednesday, and immediately launched his own investigation into the flat.
The appointment of the new adviser paves the way for the publication of the latest register of ministerial interests, which could contain details of any donations to fund the flat.
However, the Prime Minister’s official spokesman said Mr Johnson will remain the “ultimate arbiter” of whether the ministerial code has been broken, even if the investigation centres on himself.