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Yahoo’s former chief Marissa Mayer has warned that the threat from hackers backed by governments has increased so much that even the best-defended companies are vulnerable to new cyber attacks.

Giving testimony on Capitol Hill on Wednesday, Ms Mayer also blamed Russian agents for data losses at the internet company which were among the largest ever thefts of personal information.

“We now know that Russian intelligence officers and state-sponsored hackers were responsible for highly complex and sophisticated attacks on Yahoo’s systems,” she told the US Senate commerce committee hearing into cyber security.

“The threat from state-sponsored attacks has changed the playing field so dramatically that today I believe that all companies, even the most well-defended ones, could fall victim.”

Ms Mayer left Yahoo in the summer when wireless group Verizon bought the internet business for $4.5bn and combined it with AOL into a new company, Oath. Verizon renegotiated the price tag, slicing $350m off its original offer, after the cyber attacks were disclosed.

Since the deal’s completion Yahoo has revealed that all 3bn user accounts were affected in the 2013 breach, three times more than it had previously estimated. The company was hit by another attack the following year.

Ms Mayer said on Wednesday that Yahoo had doubled its security staff during her tenure, helping to combat a “barrage of attacks”. But “no company, individual, or even government agency is immune from these threats”, she added.

She was speaking at the latest Washington hearing into corporate failure to safeguard personal information. The current and previous chief executives of Equifax, the credit reporting group hit by a cyber attack this year, also appeared.

Paulino do Rego Barros, interim chief at Equifax, said the company was “rapidly improving” its IT security after the data breach put millions of Americans at risk of identity theft.

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