Mr Johnson was accusing the Government of "fiscal fundamentalism" over its decision to increase VAT when he was asked about Labour's policy of raising NI contributions.
The Shadow Chancellor said employers' contributions would have gone up one per cent from April, to 21 per cent, had Labour won the General Election but when asked what the current rate was he didn't reply.
Sky News' presenter Dermot Murnaghan interjected, saying: "12.8 per cent. Right, okay, I helped you out there. Just out of interest, there is still a lot to learn for you in terms of the job."
Mr Johnson replied: "There's not a lot to learn about how we tackle this deficit."
And the MP for West Hull and Hessle denied he was an "economic novice", saying that he had "boned up" on the job since his appointment by Labour leader Ed Miliband.
Co-chairman of the Liberal Democrat Treasury Committee, Stephen Williams, described the gaffe as "utterly incredible".
He added: "Labour left the country in an economic shambles. They have made no apology, they have no regrets and with a self-proclaimed novice at their economic helm, it's little wonder they still offer no alternative."
The blunder followed opinion polls that suggested Labour is on course for a comfortable win in the Oldham East and Saddleworth by-election in what would be a serious blow for the Liberal Democrats.
Despite intensive campaigning by a string of Cabinet Ministers, Nick Clegg's party trails by 17 points in two separate surveys of voters in the key marginal.
A Sunday newspaper poll put Labour on 46 per cent, the Lib Dems on 29 per cent and the Conservatives on 15 per cent.I