The RSPCA has been urged to improve the way it is run following the departure of its chief executive after just over a year in the role.
The Charity Commission said governance of the RSPCA "remains below" what is expected of "a modern charity", adding that it was concerned about the impact on public confidence.
It warned the animal welfare charity it could face "further regulatory action" unless necessary improvements were made.
The regulator's warning to the RSPCA comes after it was announced chief executive Jeremy Cooper had stepped down from his position.
A Charity Commission spokesman said: "The governance of the RSPCA remains below that which we expect in a modern charity and we are concerned about the impact on public confidence.
"This has been brought into focus by the departure of the CEO and the clear recommendations of the charity's independent governance review, which the commission requested the charity carry out.
"We have written to the charity to make clear that we expect to see a swift action plan against these recommendations, which we will formally monitor, in order to resolve these issues.
"We will consider what further regulatory action may be required should improvements not be made with the necessary urgency."
The RSPCA said on Monday that Mr Cooper had left to "pursue other business opportunities", adding that he had been "an asset to the team".
Mr Cooper said his arrival would herald a new direction for the charity and mark a departure from the fiery rhetoric used in the past when he took on the role last year.
Speaking to The Daily Telegraph at the time of his appointment, Mr Cooper said he wanted to see the number of prosecutions brought by the charity reduced and virtually eliminated for fox hunting.
The RSPCA famously prosecuted the Heythrop, former prime minister David Cameron's local hunt, with success coming after huge sums were spent.
The charity also attracted negative publicity for its failed prosecution of a family for alleged cruelty to its cat.
"We have made mistakes," Mr Cooper told the newspaper.
"We have to be honest about that. We have to admit that and acknowledge that.
"The important thing when you make mistakes is what you do about it.
"We are going to be a lot less political. It doesn't mean we won't stand up for animals. But we are not a political organisation."