The article in the left-wing weekly predicted Mr Blair would be ousted in a "Palace coup" and probably replaced by Robin Cook, the shadow Foreign Secretary.
Trouble-stirring and malicious stuff: so who is Cassandra? Suspicion naturally fell in turn on a number of well-known members of Labour's Awkward Squad but on closer examination most chuckling suspects were quickly allowed to escape to freedom.
Early candidates included Austin Mitchell and Tony Banks, the two best jesters on the Labour benches. Mr Mitchell recently wrote an article in the New Statesman likening Mr Blair to Kim Il-Sung, Korea's late strongman, and is so Eurosceptic that he is danger of declaring UDI for Grimsby, his constituency. Mr Banks is a general troublemaker who has come out in favour of legalisation of cannabis - but eschewing the drug himself - and is so unable to keep his mouth shut that his periods on the front bench have always been cut short.
Examination of the article ruled both these out. It was far too boring and long-winded. "LONG-WINDED," they all cried. What about Neil Kinnock? Oh, he's no longer an MP. So suspicion fell on other recent noise-makers. Paul Flynn, another pro-cannabis man, was a happily obscure MP until recently when he criticised Mr Blair for sending his child to a grant- maintained school. But Mr Flynn could hardly describe himself as senior, having entered Parliament in 1987.
The braver manhunters decided to approach His office. A spokeswoman for the Labour leader said: "We would rather nail the culprit. The only thing that is giving this thing legs is the mystery."
Sources close to Mr Blair yesterday pointed the finger of suspicion at Brian Sedgemore, the disaffected MP for Hackney South. Mr Sedgemore got into deep water for writing under the name Justinian in Private Eye and a diary under his own name in the New Statesman in the 1980s. Recently, he published a book containing salty observations on his parliamentary colleagues, again under his own name.
But Mr Sedgemore denied authorship, and friends say that the views in the article are certainly not his. Denzil Davies, the former front-bencher and anti-EMU campaigner, was ruled out for the same reason.
Yesterday, another possible suspect was John Garrett, MP for Norwich South. Like Cassandra, Mr Garrett is deeply sceptical about devolution and especially regional government for England. Moreover, he knows Mark Seddon, the editor of Tribune. The fact that Cassandra does not appear to be a Euro-sceptic would also fit his profile as a member of the European Movement.
But Mr Garrett vehemently denied the suggestion yesterday: "It's certainly not me, I swear."
The choice of name suggests a failure of judgement by the culprit. Cassandra had the fate of being expert at prophesying but of never being believed. And she was murdered by Clytemnestra for being a "pain in the neck", according to The Independent's resident Greek scholar.