The Sun has become the first British newspaper to publish the photos of a naked Prince Harry taken in Las Vegas.
Its owner News International said it was making the move despite warnings from the Royal Family's lawyers that it would be an invasion of his privacy.
The Sun said the images were widely available around the world, its readers had a right to see them and freedom of the press was being tested.
The pictures emerged from a private weekend the prince spent with friends.
The two photos of the prince and a naked woman in a hotel room are believed to have been taken on a camera phone last Friday.
They first appeared on US website TMZ earlier this week.
In Friday's Sun, under the headline "Heir it is", the paper says: "Pic of naked Harry you've already seen on the internet".
David Dinsmore, managing editor of the Sun, said the paper had thought "long and hard" about publication and added: "For us this is about the freedom of the press.
"This is about the ludicrous situation where a picture can be seen by hundreds of millions of people around the world on the internet but can't be seen in the nation's favourite paper read by eight million people every day.
"This is about our readers getting involved in the discussion with the man who is third in line to the throne - it's as simple as that."
In an editorial, the newspaper said the pictures represented a crucial test of Britain's free press.
'Decision for editors'
St James's Palace had contacted the Press Complaints Commission (PCC) on Wednesday because it said it had concerns about the 27-year-old prince's privacy being intruded upon, in breach of the editors' code of practice.
The palace said it had heard a number of UK newspapers were considering using the pictures, although none had until now.
In reaction to the Sun's decision, a palace spokesman said: "We have made our views on Prince Harry's privacy known. Newspapers regulate themselves, so the publication of the photographs is ultimately a decision for editors to make.
"We have no further comment to make either on the publication of the photographs or on the story itself concerning Prince Harry's private holiday in Las Vegas."
TMZ reported that Harry had been pictured in a group playing "strip billiards".
The Sun said in a statement that in publishing the photos it was not making any moral judgement about the prince's activities.
It said: "He often sails close to the wind for a royal - but he's 27, single and a soldier.
"We like him. We are publishing the photos because we think Sun readers have a right to see them. The reasons for that go beyond this one story."
It added: "There is a clear public interest in publishing the Harry pictures, in order for the debate around them to be fully informed.
"The photos have potential implications for the prince's image representing Britain around the world. There are questions over his security during the Las Vegas holiday. Questions as to whether his position in the Army might be affected.
"Further, we believe Harry has compromised his own privacy."
The non-publication of the photos by British newspapers despite their publication elsewhere had prompted a debate about the impact the Leveson Inquiry was having on press behaviour.
Former News of the World executive editor Neil Wallis said it showed British newspapers had been "neutered".
The Leveson Inquiry was set up to investigate the practices and ethics of the press following the phone-hacking scandal.
Commons culture, media and sport select committee chairman John Whittingdale said of the Sun's decision: "The fact that [the photos] happened is well known. How the public interest is served by doing this is not clear."
Ex-Deputy Prime Minister John Prescott said the Sun had shown "absolute utter contempt" for the law and the Leveson Inquiry.
Former Sun editor Kelvin Mackenzie told Newsnight that in his opinion the decision to publish the pictures could not have been made without News International boss Rupert Murdoch's consent.
Mr Mackenzie added: "I'm unsure why the establishment hate newspapers so much but what I'd like to see is editors get off their knees and start pushing back against these curtailments in what will eventually, I promise you, lead to the closure of newspapers.
"People should stop worrying about privacy and start worrying about what free speech will mean to this country if the Levesons and the Camerons of this country have their way."
Both the Daily Mirror and the Independent said they had not published the photos because they considered that they breached the prince's privacy.