The Maleficent actress, who is now a UN ambassador and mother-of-six, claims the newspaper 'grossly violated' her privacy.
George Clooney has accused the Mail Online of 'fabricating' a story about his fiances mother opposing his marriage.
George Clooney may be mulling a bid for the governorship of California - could the Gravity star become the latest celeb-turned-politician?
George Clooney has described the Daily Mail as the "worst kind of tabloid", as he rejected the Mail Online's apology for a story about his upcoming marriage.
The Mail Online apologised to the actor after publishing an article claiming his fiancee's mother opposed their marriage on religious grounds.
Clooney said the Mail's claim that the original story was "not a fabrication" was not true, and that the paper was either "lying originally or they're lying now".
Writing in the USA Today, Clooney said: "I thank the Mail for its apology. Not that I would ever accept it, but because in doing so they've exposed themselves as the worst kind of tabloid.
"One that makes up its facts to the detriment of its readers and to all the publications that blindly reprint them."
The Mail Online website has apologised to George Clooney and promised a full investigation into a story that claimed his fiance's mother did not approve of his upcoming marriage.
In a statement they said: "The MailOnline story was not a fabrication but supplied in good faith by a reputable and trusted freelance journalist."
They added: "We only became aware of Mr Clooney’s concerns this morning and have launched a full investigation. However, we accept Mr Clooney’s assurance that the story is inaccurate and we apologise to him, Miss Amal Alamuddin and her mother, Baria, for any distress caused."
In an earlier article for the USA Today newspaper, the Gravity star accused the Daily Mail newspaper of printing a "completely fabricated" story about his fiance's mother opposing their forthcoming marriage for religious reasons.
Legal colleagues have praised the "utterly wonderful" human rights lawyer Amal Alamuddin, who has become engaged to Hollywood heartthrob George Clooney, and offered their congratulations.
Eminent lawyer Geoffrey Robertson, who is the joint head of chambers, said: "Amal Alamuddin is a brilliant and passionate defender of human rights who has put in the enormously hard work necessary to improving them. She is respected and admired by all her colleagues."
The barrister, who was educated at Oxford University, has represented WikiLeaks founder Julian Assange, and has been described as a "passionate defender of human rights". Beirut-born Alamuddin, 36, is a member of Doughty Street Chambers in London, having joined in 2010 to complete her pupillage.
Hollywood star George Clooney has reportedly got engaged to British lawyer Amal Alamuddin.
The 52-year-old star is said to have shown the engagement ring to friends over the weekend.
People magazine reports a source close to the couple saying:
"George and Amal are trying to keep things very low-key but they also aren't really trying to hide this, it doesn't seem. I think it's like they want the people they love to know that this is real, that they plan on being together forever."
Alamuddin, 36, has represented high-profile clients including the founder of Wikileaks, Julian Assange.
American actress Eliza Dushku and Glee star Harry Shum Jr have sent their support to George Clooney after he was arrested outside the Sudanese embassy.
George Clooney. Don't know the full details of his arrest but from what I have read, he's a stand up guy.