News of the couple's engagement was confirmed in April when Alamuddin's legal practice in London issued a statement congratulating them.
The banns, or formal notice of the wedding, were posted at Kensington and Chelsea Register office in southwest London from last month until Thursday, the spokesman said.
The couple would have had to visit the office and pay a fee of 35 pounds ($59) each before signing the document which was then displayed for 16 days.
Two-time Oscar winner Clooney, 53, has been married once before but since his 1993 divorce from Talia Balsam has remained one of Hollywood's most eligible bachelors and had vowed never to remarry.
Alamuddin has advised United Nations former secretary-general Kofi Annan on Syria, represented Ukraine's ex-prime minister Yulia Tymoshenko in challenging her detention before the European Court of Human Rights and has also represented WikiLeaks founder Julian Assange in extradition proceedings.
Photos of the notice, published by the Daily Mirror, show 36-year-old Alamuddin's occupation as "barrister" and her status as "single". Clooney, listed under his full name of George Timothy Clooney, is described as "actor and director", with his marital status "previous marriage dissolved".
The document names the location of the wedding as Italy but does not give a date. Media have reported the ceremony will take place in September at Clooney's villa by Lake Como.
Clooney's publicist, Stan Rosenfield, said he could not reveal any information about wedding plans.
A spokeswoman for Alamuddin's law firm was not immediately available to comment.