Shadow health secretary Wes Streeting said he was “very, very sceptical” about there being a case to decriminalise the Class B drug.
A London drugs commission, led by the former lord chancellor Lord Falconer, has been set up by the Mayor, and its members are due to be announced this week. Its findings will be published next summer.
But Mr Streeting, Labour MP for Ilford North, told the Standard: “I am very, very sceptical, to put it mildly, about the case for legalisation of drugs, including drugs like marijuana, where I think there are serious health side effects.
“People know that drugs are linked to serious organised crimes. Ultimately, drugs policy is a matter for Westminster.”
Mr Khan said the commission would examine the effectiveness of British drugs laws on cannabis and the potential for reducing “drug-related harm”.
The commission was announced in May as Mr Khan visited a regulated cannabis factory in LA.
He also appeared attracted by the ability of American mayors to raise tax revenues by regulating cannabis.
City Hall does not have the power to change the law on cannabis but could influence the debate.
At the time, Mr Streeting reportedly told a WhatsApp group of Labour MPs: “Does this make it more or less likely that we win a general election?”
He told the Standard that a Labour government would take an approach “which both helps people to break drug addiction, but also comes down like a ton of bricks on drug dealers, particularly those that are using children to ferry drugs across the country”.
He added: “I’m not persuaded by the... argument that we take drugs and we sell them in licensed premises because we think we can make a big buck out of it.”
This month, research by University College London and King’s College London found adolescents were more than three times more vulnerable than adults to developing a cannabis addiction.
Polling by the Civitas think tank suggested if cannabis was legalised there would be almost 800,000 potential new users in London.
A spokesman for the Mayor said: “The Mayor established the commission to ensure there is an evidence-based debate around our drug laws, so that we can take a collaborative approach to tackling harms.”