Downing Street has described the groping allegation against Tory mayoral hopeful Daniel Korski as “very serious”.
No 10 also indicated prime minister Rishi Sunak would urge any further complainants to come forward.
Television producer Daisy Goodwin made a formal complaint after she publicly accused the former adviser of sexually assaulting her during a meeting in Downing Street a decade ago.
Korski, who denies what he calls a “baseless” claim, has been resisting pressure to quit the race to be the Conservative candidate to lead London, despite support draining away.
Goodwin, who wrote the hit TV show Victoria, alleged it took place while he was a special adviser to then-prime minister David Cameron and has complained to the Cabinet Office.
Downing Street declined to say whether Sunak believes Korski is a suitable candidate or if he should suspend his campaign, but did say “conclusions shouldn’t be drawn on until the processes are followed through”.
Sunak’s press secretary said: “Obviously these allegations are very serious.
“They are allegations that have obviously been denied by Daniel Korski himself. They should be handled in the proper way.”
Goodwin has said she has been contacted by other women with “some very interesting stories” that make her “feel entirely justified” about making the allegations against Korski.
Asked if the PM would encourage anyone with complaints against former staff members to come forward, his press secretary said: “Yes, of course.”
The press secretary declined to “get into ‘he said, she said’” when asked who Sunak believes, but said he was not aware of the allegation before it was made public.
Downing Street also would not set out whether it would fall within the remit of the Cabinet Office or the Conservative Party to investigate the allegation.
Cabinet office complaint
But the Prime Minister’s official spokesman suggested a “serious allegation” would be looked at by the “appropriate authority” regardless of the passage of time.
Goodwin confirmed she had submitted a complaint to the Cabinet Office, although she said it was “harder than trying to get a telephone engineer”.
Korski has said it is “categorically” untrue that he groped Goodwin and has pledged to stay in the race, as he faces two other challengers to be the Tory candidate for London mayor.
The Cabinet Office declined to confirm whether a formal complaint has been made. The Conservative Party has said it is not investigating the allegation.
A Tory spokesman said: “The Conservative Party has an established code of conduct and formal processes where complaints can be made in confidence.
“The party considers all complaints made under the code of conduct but does not conduct investigations where the party would not be considered to have primary jurisdiction over another authority.”
By Sam Blewett, Dominic McGrath and David Hughes, PA Political Staff