Prince William's ex-girlfriend Kate Middleton was "hounded" by the paparazzi and the press watchdog took too long to protect her, MPs have said.
In a report on press regulation, the committee said Miss Middleton had been a victim of "persistent harassment".
It said the Press Complaints Commission (PCC) had been "less than impressive" and should have intervened sooner.
The PCC said it had kept in touch with Miss Middleton's lawyer and would not have intervened without her consent.
Prince William split with Miss Middleton in April, after six years of intense media pressure since they met at St Andrews University, Fife, in 2001.
However, there has been recent press speculation the couple are back together and this was further fuelled by Miss Middleton's presence at the Concert for Diana at Wembley earlier this month.
Media interest in the couple reached new heights in January, amid mounting speculation they were about to get engaged, when a media scrum formed outside Miss Middleton's home on her 25th birthday.
Days later the PCC circulated a letter from her solicitors, saying Miss Middleton was being harassed and a formal complaint would be made if paparazzi pictures continued to be used.
That month police officers were called to a London nightclub to stop photographers getting too close to the couple.
And, in April, Miss Middleton settled a harassment complaint over a close-up photograph published by the Daily Mirror. The paper admitted "we got it wrong".
When the settlement was announced the PCC issued a warning over press treatment of Miss Middleton.
But MPs on the Culture, Media and Sport Select Committee said the PCC "took too long to act to protect Kate Middleton from clear and persistent harassment".
The report said: "In the case of Ms Middleton, harassment was evident, yet photographs taken by the paparazzi continued to appear in national and regional papers. We see no plausible public interest defence.
"We conclude that editors, in failing to take care not to use pictures of Kate Middleton obtained through harassment and persistent pursuit, breached... the Code of Practice."
But the report said the PCC seems to have waited for a complaint when "it should have intervened sooner".
The letter from Miss Middleton's solicitors was circulated "long after the worst abuses had occurred," the report said.
But the PCC said the committee had been "unfair" in not referring to the conversations it had with Miss Middleton's lawyer during that period.
PCC director Tim Toulmin said it was up to the lawyer to decide when they wanted a letter to be circulated. "It is unfair to criticise us for not acting when we were not being asked to act," he said.
"But they paid us a backhanded compliment by recognising that we are effective when we do act."
The committee concluded that press self-regulation should continue, but highlighted the Middleton case as one of several "recent lapses in standards".
These also included News of the World reporter Clive Goodman being jailed for illegally accessing voicemail messages from phones belonging to members of the Royal household.
But the select committee says the current system of self-regulation should continue and was "infinitely preferable to the alternatives".
Committee chairman John Whittingdale said the PCC and editors should be vigilant in ensuring all journalists were "abiding by both the spirit and the letter of the code."
He told BBC Breakfast that Ms Middleton had been subjected to "really intolerable harassment".
"Particularly one morning on her birthday when she opened her front door to be confronted by a bank of press photographers and TV cameras and was then pursued down the street," he said.
"Now that quite clearly is a breach of the Press Complaints Commission code and we are critical of the Press Complaints Commission for not acting sooner to put a stop to it."
PCC chairman Sir Christopher Meyer told BBC Radio 4's Today programme it was a "pretty good report overall" because it laid down two landmark judgements coming out "very categorically against" both a privacy law and statutory regulation of the press.
He said it was "inevitable" the PCC's code would be breached but he wanted to reduce the number to the absolute minimum possible.
"What I want is the highest possible standards based on the code of practice," he said.