News International's chief executive has said The Sun must start charging readers of its website as the company confirmed it would introduce a paywall later this year.
The Sun is following the Telegraph, which announced on Tuesday it would implement a subscription system for online access.
Mike Darcey, chief executive of publisher News International, said the firm had no choice but to ask readers to pay for the online version of the red-top which is threatening the tabloid's circulation and revenues.
"This decision comes from a deep-seated belief that it is just untenable to have 2.4 million paying 40p for the Sun at the same time as a bunch of other people are getting it for free," he told The Guardian.
"The Sun website is a fantastic website and sometimes it is a better product. I have to believe that we are all suffering, to a greater or lesser degree in the print world because of that, that part of those people are saying day in, day out 'why am I paying for the Sun when I can get it for free'."
News International promised readers would be offered a "bigger and better experience than they have ever had before" when the new system kicks in. Its announcement comes after the company struck a deal in January to show Premier League highlights online and on mobile phones and tablets.
A News International spokesman said: "Later this year the pay model will be applied to The Sun across every platform. We will be offering our valued Sun readers a bigger and better experience than they have ever had before - one that in addition to FA Premier League clips will offer a full and attractive subscription model across digital and print."
The Telegraph will offer visitors to its website a choice between two different packages after using up a quota of 20 free stories a month. Readers will be offered a free month's trial before being asked to commit to a subscription.
The paywall is being introduced to the UK site after a similar "metered" system was trialled on the international version of the website.
Telegraph Media Group said the international version had seen nine out of 10 people going on to subscribe after taking up the free trial.