He told bankers to "palliate their guilt" by donating to his Mayor's Fund, but it turns out that Boris Johnson is far less keen on giving to charity himself.
"Strongarmed" by his then aide Nick Boles to give up a fifth of his £250,000 Daily Telegraph earnings to charity Boris is said to have exclaimed:
"It's outrageous! I've been raped! I've been raped!"
Boris apparently bitterly resents what he calls this "Boles Tax" and has so far only donated a fraction of the sums he publicly committed to.
According to a new biography written by his former Telegraph colleague Sonia Purnell, Boris has:
"donated only a total of £20,000 over three years (compared to the £75,000 pledged) to fund six bursaries for a sports journalism course at the College of Communications, with another final sum of £10,000 expected. He has also not given 'nearly as much' as £75,000 to a new charity set up by Friends of Classics to support Latin and Greek teaching in state schools"
Boris has also apparently made no commitment to continue these donations if re-elected.
You can read more about this and other embarrassing claims in Just Boris: The Irresistible Rise of a Political Celebrity which can be ordered from Amazon over here.