The British government's proposed new internet regulations have caused alarm among critics who say the plans threaten freedom of speech.

The new rules, suggested in a white paper published on Monday, and the creation of a regulator to enforce them, are part of recommendations that attempt to tackle the spread of material related to terrorism, child abuse, self-harm and suicide on the internet.

The regulator would oversee online companies that "allow users to share or discover user-generated content or interact with each other online."

The Department for Digital, Culture, Media and Sport says the independent watchdog would write and uphold a "code of practice" for tech companies. Senior managers could be held liable for breaches, with a possible tax on the industry to fund the regulator.

Culture Secretary Jeremy Wright has dismissed concerns that the new regulator would harm press freedom, saying that the new rules would not affect "journalistic and editorial content".

Critics have warned the legislation could threaten freedom of speech. Jim Killock, the executive director of the Open Rights Group, told the Guardian: "The government's proposals would create state regulation of the speech of millions of British citizens. We have to expect that the duty of care will end up widely drawn, with serious implications for legal content that is deemed potentially risky, whether it really is or not."

Freedom of expression group Article 19 has strongly opposed a "duty of care" being imposed on internet platforms.

It said this would "inevitably require them to proactively monitor their networks and take a restrictive approach to content removal", adding: "Such actions could violate individuals' rights to freedom of expression and privacy."

On Monday, government ministers sought to reassure critics. Home Secretary Sajid Javid said: "Press freedom is absolutely sacrosanct in this country" and that "there is nothing in this white paper that challenges that in any way whatsoever".

In a letter to the UK's Society of Editors, addressing press industry concerns, Wright said the new regulator would have a "legal duty to pay due regard to protecting users' rights online, particularly their privacy and freedom of expression", adding: "We are clear that the regulator will not be responsible for policing truth and accuracy online."

"The government absolutely upholds the core principle of freedom of expression recognizing the invaluable role a free press plays in our culture and democratic life," Wright said.

He said he was keen to work closely with the press "to ensure our proposals are effective, proportionate and do not have unintended consequences".

He added: "A vibrant, independent, plural and free press that is able to hold the powerful to account is essential to our democracy."

The government, which is consulting on the proposals, now faces pressure to make amendments before they become law.