Nadine Dorries is under mounting pressure to formally quit as an MP after she apparently delayed the move in order to make life difficult for Rishi Sunak.
The former cabinet minister - a key ally of Boris Johnson - announced last Friday that she was resigning “with immediate effect” after being blocked from receiving a peerage.
However, she has since failed to go through the formal process of standing down to trigger a by-election in her Mid Bedforshire seat.
Downing Street had hoped to hold all three by-elections on the same day before parliament rises for the summer recess next month.
But it is understood Dorries now plans to delay standing down, meaning the prime minister would have to face another tricky by-election in the autumn.
The PM’s spokeswoman today said: “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 believes the people of Mid Bedfordshire deserve proper representation in this House and he looks forward to campaigning for the Conservative candidate in the by-election.”
Tory MP Aaron Bell told Radio 4′s World At One programme: “I think it would be good for her constituency in Mid Bedfordshire… if they could have proper representation, because Nadine’s barely been seen in parliament these last six months while she’s been earning money on telly.”