Mark Menzies has "agreed to relinquish the party whip" but denies the allegations.
Better economic news fails to turn around the PM's fortunes.
"I genuinely don't know how she manages to do it. It's extraordinary."
The former prime minister's short stint in No.10 is mainly remembered for her disastrous mini-budget.
Former Tory adviser Luke Tryl said it would be "historically unusual" if Keir Starmer does not end up in No.10.