After insisting that he would only back a candidate "at the time I decide to stand down", Mr Blair referred to his chancellor's performance at the despatch box yesterday as an "absolute and comprehensive drubbing" delivered to the Tory front bench.
Mr Brown faced a vote of no confidence over his 1997 decision to scrap tax relief on pensions, but survived thanks to Labour's parliamentary majority.
"I would be rather more worried about the leadership potential on his side than this," Mr Blair said, to uproarious laughter from the backbenches.
Mr Cameron pursued his line of questioning, crowing that "he will not endorse the chancellor".
"On this side of the house we know why we don't want the chancellor," the Conservative leader added, saying that in addition to ruining Britain's pensions policy Mr Brown "never says sorry" and is "impossible to work with".
But it was the prime minister who had the last laugh, taking advantage of Mr Cameron's role as a special adviser to then-chancellor Norman Lamont in 1992 when Britain fell out of the exchange rate mechanism.
Referring to Mr Cameron, he said: "He had something to do with the British economy once didn't he? Back in 1992? We remember what's he delivered for the British economy - a bit-part on Black Wednesday."
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