Mark Zuckerberg made the disclosure during his second day of being questioned in Washington.

In a related development, the political consultancy at the heart of the affair has announced its acting chief executive is stepping down.

He also revealed that his firm was exploring whether to take action against the University of Cambridge.

The institution is where the researcher Aleksandr Kogan, who had collected and sold personal data to Cambridge Analytica, was based.

"What we found now is that there's a whole programme associated with Cambridge University where... there were a number of other researchers building similar apps," Mr Zuckerberg said.

"So, we do need to understand whether there was something bad going on at Cambridge University overall that will require a stronger reaction from us."

The university has reacted by saying it would be "surprised" if Mr Zuckerberg was only now aware of the work.

"Our researchers have been publishing such research since 2013 in major peer-reviewed scientific journals, and these studies have been reported widely in international media," it added.

"These have included one study in 2015 led by Dr Aleksandr Spectre (Kogan) and co-authored by two Facebook employees.

"We have found no evidence that University researchers are improperly gathering personal data."

Mr Zuckerberg had earlier apologised for having failed to check in 2015 that Cambridge Analytica had deleted information gathered about millions of Facebook users.

Instead Facebook let the political consultancy self-certify that it had destroyed the records, which it said had been acquired in violation of the social network's rules.

"We have a responsibility to make sure what happened with [app developer] Kogan and Cambridge Analytica doesn't happen again," Mr Zuckerberg said.

The 33-year-old added that Facebook's audit of other apps would take many months to complete. But he said that the firm had seen no evidence as yet that Russia or China had attempted to scrape people's information.

Cambridge Analytica has denied breaking the law and said it did indeed delete the data.

The company has also announced that its acting chief executive Dr Alexander Tayler is stepping down from the post to resume his previous position as its chief data officer.

Alexander Nix, who had been in charge until March, remains suspended.

Mr Zuckerberg's four hour appearance before the House Committee on Energy and Commerce followed a five hour session in front of two Senate committees yesterday.

During a testy early exchange, he declined to give a commitment to change all users' default privacy settings to collect the minimum amount of personal information.

"This is a complex issue," Mr Zuckerberg said.

"That's disappointing to me," responded Democratic congressman Frank Pallone.

Elsewhere in the hearing, Mr Zuckerberg acknowledged that he believed it was "inevitable" that the internet would need new regulations.

"My position is not that there should be no regulation, but I also think you have to be careful about what regulation you put in place," he said.

The Facebook chief also faced fresh accusations from Republican congressman Steve Scalise that the News Feed's algorithm was discriminating against conservative news and content in favour of liberal posts.

"There is absolutely no directive in any of the changes that we make to have a bias," responded Mr Zuckerberg.

"To the contrary our goal is to be a platform for all ideas."

Another Republican pulled a surprise by displaying photos of illegal adverts for opioid drugs that he said had been live on Facebook yesterday.

"Facebook is actually enabling an illegal activity and in so doing you are hurting people," said congressman David McKinley.

This screenshot showing adverts for Oxycodone and Buprenorphine was shown to the Facebook chief

"There are a number of areas of content that we need to do a better job of policing," Mr Zuckerberg replied, adding that he believed artificial intelligence tools would make this possible in the future.

However, several of the politicians urged Facebook to be more proactive about the matter.

Mr Zuckerberg also faced questions about the illegal trade of ivory in closed Facebook groups.

In another combative exchange, the business leader was questioned about the data his firm collected on people who had never signed up to his service.

Mr Zuckerberg said this was done for security purposes.

But he professed not to be familiar with the term "shadow profiles" despite it having been used widely by the media during a past Facebook data privacy controversy.

And he was unable to say how many types of data were being gathered about non-members.

"You said everyone controls their data," said Democratic congressman Ben Ray Lujan.

"But you are collecting data on people that are not even Facebook users, that have never signed a consent or privacy agreement.

"When you go to Facebook's 'I don't have a Facebook account page and would like to request all my personal data stored by Facebook', it takes you to a form that says go to your Facebook page and then on your account settings you can download your data.

BBC

More about: Facebook