The Times and Sunday Times newspapers have launched redesigned websites, in preparation for introducing charges for access next month.
The sites will replace the existing combined site, Times Online.
Users will pay £1 for a day's access and £2 for a week's subscription from June in a move likely to be watched closely by the industry.
Times Editor James Harding said it was time to stop "giving away" journalism in the two newspapers.
Mr Harding said those at the News International (NI) papers were "clearly aware" that they were taking a big step.
He told BBC Radio 4's Today programme: "Our feeling is that it is time to stop giving away our journalism. That's because we feel that we are undermining the value of our journalism, undermining the value of the Times and undermining the perception that journalism and news has a value."
The Times editor admitted that the move was a risk, since many readers were likely to be lost.
NI has said it expects to lose about 90% of its online readership.
But Mr Harding remained confident that the newspapers' core readership would be prepared for web access.
"I think we are going to lose a large number of readers who are window shoppers," he said.
The media industry uses a general yardstick that about 5% of visitors to news websites are likely to pay for content.
Recent figures show that The Times and Sunday Times had about 1.22m daily users.
The BBC's Rory Cellan-Jones said the use of "paywalls" would "tell us much about the future of journalism and the willingness of readers to pay for it".
He said: "While there's been plenty of sniping from the sidelines by News International's rivals, I suspect they are all glad that someone is at least testing the waters.
"If the two papers do attract enough paying customers to start eating into the company's losses, that will give heart to the whole newspaper industry."
NI owns the Wall Street Journal, which has one of the most successful paid-for sites with about 407,000 electronic subscribers.
However, some analysts point out that the WSJ offers specialist content, and that charging for general news is a different business model.