Concerns have been raised that bloggers may face stiff libel fines under rules imposed by a new press watchdog.
Political blogger Iain Dale said he would "certainly" be covered by the regulator and the Huffington Post's Carla Buzasi said the body's remit relating to the internet was unclear.
But the government says the criteria that determines whether a publisher is liable protects "a single blogger".
Meanwhile, some newspapers are seeking legal advice on whether to co-operate.
The publishers of the Daily Mail, the Sun, the Times, the Telegraph, the Daily Star and the Daily Express said they would wait to make a decision.
The new press watchdog is to be established in England and Wales by royal charter and backed by legislation.
The new regulatory regime will replace the current system, under which the press is self-regulated voluntarily through the Press Complaints Commission.
The deal follows Lord Justice Leveson's inquiry into press ethics - held in the wake of the phone-hacking scandal.
The inquiry examined the ethics and practices which allowed journalists to hack thousands of phones. It called for a new independent press watchdog, backed by legislation to ensure it was doing its job properly.
Party leaders said the new independent regulator - with powers to demand up-front apologies from UK publishers and impose £1m fines - would protect victims of press intrusion and preserve press freedom.
But the extent to which the new regulation applies to the internet is not yet clear.
Having read the royal charter, Mr Dale wrote: "I think my blog would certainly fall under the remit. And it stinks."
"If I don't sign up and I am successfully sued, a judge would award exemplary damages against me," he said, adding that he could not risk his family's financial future.
"This is madness. All that will do is encourage people with a grudge to make a complaint in the full knowledge that they will never be held responsible for what they are doing," he said.
On Monday, Culture Secretary Maria Miller said that to be affected by the change, a "publisher would have to meet the three tests of whether the publication is publishing news-related material in the course of a business, whether their material is written by a range of authors - this would exclude a one-man band or a single blogger - and whether that material is subject to editorial control".
She said the new rules were designed to protect "small-scale bloggers" and to "ensure that the publishers of special interest, hobby and trade titles such as the Angling Times and the wine magazine Decanter are not caught in the regime".
Guidance issued by the Department for Culture, Media and Sport (DCMS) said all three of these tests would have to apply.
But "online-only edited 'press-like' content providers" - such as the Huffington Post and Holy Moly Gossip - would be relevant publishers.
"Ultimately, it is a matter for the court to decide on the definition of a relevant publisher based on assessment of the facts, in accordance with the three interlocking tests - course of business, range of authors and editorial control," said a DCMS spokeswoman.
Mrs Buzasi, editor-in-chief of the Huffington Post UK, said she was "concerned" there was confusion over who was covered, adding that she felt the agreement had been "rushed".
And Harry Cole, who writes for the Guido Fawkes political blog, said the regulatory changes had been implemented in a "chaotic fashion".
"They don't understand that [the internet] is the future. There are not necessarily going to be newspapers in 10 years," he said.
Fraser Nelson, editor of The Spectator weekly news magazine, told the BBC's PM programme his publication would not be signing up to the new regulations.
He said politicians had "created a new club" and that "looking at the membership rules, it's not something The Spectator feels like joining up to".
He added that he hoped newspapers "will not go down this route" and instead devise an alternative system.
Earlier, a joint statement signed by Associated Newspapers, News International, the Telegraph Media group and Northern & Shell said no newspaper or magazine industry representative had been involved in the cross party talks on press regulation on Sunday and that they had only seen the royal charter on Monday.
The Newspaper Society, representing local papers, said the proposals would place "a crippling burden on the UK's 1,100 local newspapers, inhibiting freedom of speech and the freedom to publish".
That was, in part, because of the "huge financial penalties for newspapers which choose to be outside the system and an arbitration service which would open the floodgates to compensation claimants", president Adrian Jeakings said.
Judges could award punitive damages against publications which refuse to sign up to a new watchdog, in the event of a court case if a complaint could have been resolved through arbitration.
Prime Minister David Cameron said he was convinced the system would work and endure: "I am confident that we've set up a system that is practical, that is workable, that protects the freedom of the press and it's a good strong self-regulatory system for victims."
He said a new system would ensure:
- upfront apologies from the press to victims
- fines of 1% of turnover for publishers, up to £1m
- a self-regulatory body with independent appointments and funding
- a robust standards code
- a free arbitration service for victims
- a speedy complaints system