Sadiq Khan regularly lobbied for sanctions against Israel when his party was in government, despite using his London mayoral campaign to claim he had consistently opposed such moves.
The Labour candidate had contacted leading politicians including David Miliband, who was then Foreign Secretary, in an effort to have measures imposed against Israel.
The JC has seen a message sent by a Parliamentary aide to a meeting of the Stop the War Coalition in February 2009 which outlines the lobbying carried out by Mr Khan.
Last month Mr Khan was challenged by Conservative MP Mike Freer to explain his position on boycotts and sanctions against Israel.
Mr Khan replied that he had “consistently” been “absolutely clear that I do not support boycotts, sanctions or divestment of Israel”.
But the message sent from his then Parliamentary office manager to a meeting of the Stop the War Coalition in Wandsworth revealed the efforts the Tooting MP had made to challenge Israel.
The letter, sent around the time of Israel’s Cast Lead operation in Gaza, explained that Mr Khan had “regularly” been in contact with then Foreign Secretary David Miliband and Middle East Minister Bill Rammell, as well as Foreign Office officials.
The office manager added: “He also wrote to David Miliband… asking that sanctions be brought against Israel.”
The message to the hard-left group added: “Sadiq's commitment to the situation in Palestine is longstanding and will, I can assure you, continue into the future.
“A fortnight ago Sadiq was a principal speaker at the launch of the Labour Friends of Palestine group, which has been set up to help to ensure the Palestinian voice is represented amongst MPs and the Labour party.”
A spokesman for Mr Khan said: “Sadiq’s position on boycotts, sanctions, and divestment of Israel could not be more clear: he completely opposes them.”
His mayoral bid has been beset by controversy in the past 48 hours. One of his leading aides resigned after being suspended by Mr Khan.
Shueb Salar was accused of posting a series of offensive messages on Twitter about women and homosexuals. He also put a picture online showing him at a firing range, with a caption joking that he could be hired as a hitman.
The Conservatives said Mr Salar’s actions raised questions about Mr Khan’s judgement.