Nadine Dorries keeps Conservative Party waiting on by-election
EPAFormer cabinet minister Nadine Dorries has not yet officially resigned as an MP, to the frustration of the Conservative Party.
The close Boris Johnson ally announced last Friday she would be standing down as MP for Mid Bedfordshire "with immediate effect".
But she is yet to officially tender her resignation - putting a by-election to replace her on hold.
No 10 said it was important for her constituents to have "certainty".
"It's obviously unusual to have an MP say they will resign with immediate effect and for that not to take place," the prime minister's press secretary added.
Ms Dorries announced she would be standing down as an MP, shortly before it was confirmed she would not become a member of the House of Lords in Mr Johnson's resignation honours list.
She has since accused Prime Minister Rishi Sunak's political team of removing her name from the list - something No 10 denies.
Nigel Adams, another MP close to Boris Johnson who was reported to be in the running for a peerage but whose name was not on the list either, has also announced he is quitting.
As Ms Dorries remains a member of Parliament, she can turn up in the House of Commons chamber to make her views known.
Anything she says would be covered by parliamentary privilege, allowing her to be outspoken on any issue, without fear of legal consequences.
Mr Johnson also announced he was leaving Parliament on Friday, ahead of a Commons report expected to accuse him of misleading MPs over the Partygate scandal, which is due to be published on Thursday.
The former prime minister has denounced the committee writing the report as a "kangaroo court" determined to "drive me out of Parliament".
Unlike Ms Dorries, both Mr Johnson and Mr Adams have officially resigned their seats, in Uxbridge and South Ruislip and Selby and Ainsty respectively.
The by-elections to replace them were triggered on Wednesday, with 3 July or 20 July the possible polling dates.
If Ms Dorries's formal resignation is tendered soon, it could still be possible for all three by-elections to be held on 20 July.
But while she keeps her party waiting, the capacity for mischief exists.
On Wednesday, the PM's press secretary said Rishi Sunak "believes the people of Mid Bedfordshire deserve proper representation in [the Commons], and he looks forward to campaigning for the Conservative candidate in the by-election".
A Conservative Party source said: "We don't know why Nadine hasn't resigned.
"But we don't want to hang around, we want to get on with those things."
The Conservatives - who are trailing Labour in national polls - wanted to conclude swift campaigns before Parliament's summer recess and for any political pain from the by-elections to be short and sharp.
But if Ms Dorries keeps her party waiting, she could force them into a potentially divisive by-election later on - for example, ahead of the autumn party conference season.