The UK and EU have warned that Twitter must comply with new content rules or face sanctions that range from fines to a total ban, as concerns were raised that hate speech will increase on the platform under the ownership of Elon Musk.

The world’s richest man has agreed a $44bn (£34bn) deal to buy the social media network, which will hand control of a platform with 217 million users to a self-confessed “free speech absolutist”.

A UK government spokesperson said companies must adhere to the forthcoming online safety bill, which requires platforms to protect users from harmful content, or face the threat of large fines and, for repeat offenders, a total ban.

“Twitter and all social media platforms must protect their users from harm on their sites. We are introducing new online safety laws to safeguard children, prevent abusive behaviour and protect free speech. All tech firms with users in the UK will need to comply with the new laws or face hefty fines and having their sites blocked.”

Thierry Breton, the EU’s commissioner for the internal market, reminded the Tesla chief executive on Tuesday that he would have to comply with the newly agreed Digital Services Act, which requires online platforms to tackle illegal content such as hate speech.

“Be it cars or social media, any company operating in Europe needs to comply with our rules – regardless of their shareholding,” tweeted Breton. “Mr Musk knows this well. He is familiar with European rules on automotive, and will quickly adapt to the Digital Services Act.”

Speaking to the Financial Times, Breton added: “We welcome everyone. We are open but on our conditions. At least we know what to tell him: ‘Elon, there are rules. You are welcome but these are our rules. It’s not your rules which will apply here.’”

He went on to warn that companies in breach of the new rules, expected to come into force in 2024, can face fines of up to 6% of global turnover and outright bans for repeat offenders.

The act will require social media platforms to allow users to flag illegal content – such as promotion of terrorism or commercial scams – in an “easy and effective way” so that it can be swiftly removed.

Musk responded to the EU and UK announcements on Tuesday, tweeting: “The extreme antibody reaction from those who fear free speech says it all.”

He later showed signs of willingness to adhere to regulations: “By ‘free speech’, I simply mean that which matches the law. I am against censorship that goes far beyond the law.

“If people want less free speech, they will ask government to pass laws to that effect. Therefore, going beyond the law is contrary to the will of the people.”

Breton spoke as human rights groups reacted to the Twitter board accepting Musk’s bid with warnings of damaging consequences if the billionaire’s commitment to free speech results in a loosening of restrictions on harmful content.

Amnesty International said it was concerned about any decision that Twitter may take after Musk’s takeover to erode enforcement of the policies and mechanisms designed to moderate hate speech online.

“The last thing we need is a Twitter that wilfully turns a blind eye to violent and abusive speech against users, particularly those most disproportionately impacted, including women, non-binary persons, and others,” Michael Kleinman, director of technology and human rights at Amnesty International USA,

Deborah Brown, a digital rights researcher and advocate at Human Rights Watch, said even small changes to the platform could have a devastating impact.

The UK is also introducing a tougher regulatory regime for digital platforms with the online safety bill, which requires businesses such as Twitter and Facebook to protect users from harmful content and carries the threat of fines of up to 10% of global turnover for breaches.

Imran Ahmed, chief executive of the Center for Countering Digital Hate, a campaign group, said: “The UK and the EU are going to have tools to deal with this. Twitter is going to find it very difficult to operate in the UK if it tries to operate on a completely free speech, zero-rules basis.”

Twitter co-founder Jack Dorsey has backed Musk takeover of the micro-blogging platform, describing the billionaire as “the singular solution I trust”.

The 45-year-old, who co-founded the company in 2006 and floated it on the New York Stock Exchange in 2013, said it has been “owned” by Wall Street and that Musk’s deal struck on Monday to take it private was the “correct first step”.

However, Dorsey, who stepped down as Twitter chief executive in November and will receive a $978m payout for his 2.4% stake when the deal is completed later this year, said that ultimately “in principle I don’t believe anyone should own or run Twitter”.

Parag Agrawal, who took over from Dorsey as chief executive, has told staff that their jobs are only safe for about the six months it will take to get the deal done.

Donald Trump, who was permanently banned from Twitter after the US Capitol riots last January, has said he would not return to the platform even if Musk allowed him. Musk has said he favours temporary “time-outs” for users who breach Twitter’s policies, instead of outright bans.