Sir Christopher Meyer "destroyed his reputation" with his memoirs of his time as Britain's Washington ambassador, Jack Straw said.
Mr Straw said the Meyer book was "tittle tattle"
The credibility of the Press Complaints Commission, which Sir Christopher heads, was also damaged, said Mr Straw.
But the book was mostly "tittle tattle" so ministers had not been able to ban it, the foreign secretary added.
Sir Christopher told MPs in December he had "played by the rules" and gained proper clearance for the book.
The memoir, DC Confidential, was serialised in two national newspapers and detailed his time as British ambassador to Washington.
It dubs several Cabinet ministers "political pygmies" and says that Mr Straw "took a long time to find his feet" as Foreign Secretary.
The former ambassador was strongly criticised by the Commons public administration committee in December, who accused him of damaging the trust between ministers and civil servants.
Giving evidence to the same committee on Wednesday, Mr Straw said he had not been worried about being "embarrassed" by the material in the book about himself.
But he thought Sir Christopher had been "unprofessional" in breaking confidences with ministers.
Asked why Sir Christopher's book had not been banned, when a book by Sir Jeremy Greenstock, the UK's ambassador to Baghdad in the run up to the Iraq war had, Mr Straw said: "Meyer's book is mainly tittle tattle."
"Yes, if you pitch your book at that sort of level, the chances are that it won't be subject to legal action. But what has happened in the case of Meyer is that he has destroyed his reputation."
He added: "The guy has been completely ostracised. He has also raised huge questions about the credibility of the press complaints commission.
"The legacy of his publication and his betrayal is a very substantial one and a very poor one for him."
In his evidence to the committee in December, Sir Christopher told the MPs he believed he had gained proper clearance before publication.
But he added there was "something very wrong" with the way the rules governing breaches of confidence by former diplomats are applied.
In his evidence on Wednesday, Mr Straw stressed he did not want to ban diplomats from writing memoirs, citing a recent book about bird watching - A Diplomat and his Birds by Andrew Palmer - as an example of a book it would be "absurd" to prevent.
But he said the British system of a permanent civil service - as opposed to the US system where civil servants are politically appointed - would "collapse" if civil servants were allowed to betray ministerial confidences in print.
Mr Straw said he planned to write his own memoirs when he retired from public life but said he would "ensure that he followed the rules".
Former Number 10 spin doctor Alastair Campbell has also written to the committee saying he would stick to the rules when he published his memoirs, Mr Straw added.
He said he would back moves - being considered by the committee - to beef up the rules governing civil service memoirs.