Boris Johnson's appointment as Foreign Secretary is "a gamble that might pay off if he can reinvent himself" and ditch his celebrity status, according to Sir Malcolm Rifkind.
The Conservative politician, who served as foreign secretary between 1995 and 1997, insisted Mr Johnson was not a Donald Trump figure, but rather a "civilised and intelligent person".
The former London mayor faced a wave of criticism when Theresa May handed him the key role in her Cabinet last week.
Speaking on the BBC's Sunday Politics Scotland programme, Sir Malcolm said: "It's a gamble that might pay off if he can reinvent himself.
"I think in all seriousness Boris is an extremely intelligent and able guy, he is not a Donald Trump figure.
"He's a very civilised and intelligent person, who could be a very good Foreign Secretary.
"However, he has made his name as a celebrity, and he's got to choose.
"You can't be a foreign secretary carrying out the rather sober serious difficult problems of international diplomacy and at the same time expect to continue to be the kind of celebrity as Boris is."
On a lighter note, Sir Malcolm recommended Mr Johnson could start his reinvention with a hair cut and by "tucking his shirt into the back of his trousers on a regular basis".
He added that the Foreign Secretary would also have to quickly demonstrate he is in command of UK policy objectives in difficult areas such as the Middle East and Russia.
"To be much more serious about it, I think he has got to adopt a dialogue, a conversation, that doesn't just get a nice enjoyable headline that cheers us all up on a Monday morning," he said.