Lawyers for Cherie Blair confirmed they have issued a claim on her behalf, understood to be against News Group Newspapers.
Graham Atkins, of Atkins Thomson, said: "I can confirm that we have issued a claim on behalf of Cherie Blair in relation to the unlawful interception of her voicemails."
In November, Tony Blair's former communications director Alastair Campbell told the Leveson Inquiry into press standards that he believed a story the Daily Mirror published about Cherie Blair's pregnancy in 1999 may have come from hacking.
He admitted he had "no evidence" that journalists intercepted either Cherie Blair's voicemails or those of her lifestyle consultant Carole Caplin, but queried the source of a number of articles about the former PM's wife.
"During various periods of the time that we were in government, we were very, very concerned about how many stories about Cherie and Carole Caplin were getting out to different parts of the media," he told the inquiry.
"I had no idea how they were getting out. In relation to not just Carole, and not just Cherie, but all of us who were involved in the government at that time, all sorts of stuff got out.
"Some of it may have got out because people who were within the government were putting it out there. Perhaps. That does happen.
"But equally there were all sorts of stories where you would just sit there scratching your head thinking, 'how the hell did that get out?'."
But in January Sunday Mirror editor Tina Weaver told the inquiry that the story in the Daily Mirror came from public relations guru Max Clifford.
She said: "The information came in to the then editor, Piers Morgan, and I was his deputy and he asked me to write it.
"He purchased it from Max Clifford, I think that's a matter of record, and he told Mr Morgan where he received the original information from, I believe."
At least 50 claims against the News of the World have now been settled, and the case of singer Charlotte Church, with her parents, James and Maria, is due to begin at London's High Court on Monday.
The two-week trial before Mr Justice Vos will not just consider Church's claim that 33 articles in the now-defunct News of the World were the product of hacking into the family's voicemails - but also set out a compensation framework for future cases.
News of Cherie Blair's legal action comes just days ahead of the publication of the first Sun on Sunday by News International.
Speculation about a Sunday edition of the biggest-selling UK daily newspaper had been mounting since the closure of the News of the World after the hacking scandal.
The announcement finally came at the weekend, amid concerns the tabloid would suffer the same fate as its sister paper after the arrests of 10 current and former senior reporters and executives since November over alleged corrupt payments to public officials.
Rupert Murdoch flew to the UK last Thursday and was said to be staying to oversee the launch of his empire's newest newspaper.
Tom Watson MP, who sits on British parliament's culture, media and sport select committee, said if Cherie Blair had been a victim of hacking, it was a "significant milestone" in the scandal.
He said: "If true, this is clearly a significant milestone in the Rupert Murdoch hacking saga.
"If anything shows that Rupert Murdoch is a fair weather friend it is the fact that his operatives would target the wife of a sitting prime minister.
"I am sure Mr Blair would now wish to finally go on the record and condemn Rupert Murdoch for failing to deal with industrial-scale criminal wrongdoing in his company."
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