The two newspaper groups could face a total bill for phone-hacking of up to £1bn, Hacked Off said earlier this year.
Dozens of celebrities have settled claims with the Mirror group , including actor Hugh Grant , while Sir Elton John, Elizabeth Hurley and Heather Mills settled claims against News Group Newspapers earlier this year.
Paul Connew, former deputy editor at the News of the World and the Sunday Mirror, told the BBC the duke was “determined to attack the popular press” in suing over “allegations which date back about 15 years.”
Mr Connew suggested that the duke had launched the claim following “the rather over the top statement earlier this week attacking the press as a whole”.
He added: “Prince William has a more mature take of the press and I expect that in the long run Prince Harry could come to regret this.”
Media lawyer Mark Stephens said royals “rarely” take legal action because it can be a “high-risk strategy”.
He told the BBC that the processes of legal disclosure of information between lawyers and then the royals being cross-examined could take them to “places they don’t really want”.
“So it is a high-risk strategy because things are outside their control.
“But it’s also a high-risk strategy for the editors who are going to have to give evidence, too.”
The duke also accused the paper of misleading readers when it published the private letter regarding his wife, by strategically omitting paragraphs, sentences and specific words “to mask the lies they had perpetrated for over a year”.
“Put simply, it is bullying, which scares and silences people. We all know this isn’t acceptable, at any level,” he said.
The Mail on Sunday spokesperson said: “We categorically deny that the duchess’s letter was edited in any way that changed its meaning.”
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