The record was Bohemian Rhapsody by Queen. It was replaced by ABBA’s Mamma Mia in January 1976.
Bohemian Rhapsody’s nine consecutive weeks at No.1 at the end of 1975 was the joint third UK total at the time, trailing David Whitfield’s ‘Cara Mia’ (10 weeks, 1954) and yodeling country star Slim Whitman’s ‘Rose Marie’ (11 weeks, 1955).
It was eventually supplanted by ABBA’s Mamma Mia, a title curiously contained in Bohemian Rhapsody’s lyrics.
Bohemian Rhapsody contains the famous "mamma mia, mamma mia, mamma mia let me go" line.
But a more striking (and more bizarre) answer arises from 1959 when Emile Ford & The Checkmates' "What Do You Want To Make Those Eyes At Me For" unseated Adam Faith's "What Do You Want" from number one. It's particularly odd as, during the transition, the tracks tied for the top spot and shared the position for one week!
But there are other examples, albeit less dramatic (ie. one-word titles), of this phenomenon. See Everyhit.com's Lyrical Connections Between Successive Number 1s to find out more.
Of interest, NME.com shares 20 Things You Probably Never Knew About ‘Bohemian Rhapsody’.