Sadiq Khan has said the quickest way for the Government to achieve economic growth is to rejoin the European single market, as he risked a fresh row with Sir Keir Starmer.
The London Mayor said the public should not "pretend" Liz Truss is interested in growth, despite her party conference speech on Wednesday, because she has not suggested closer integration with Europe.
Speaking on LBC on Thursday, Mr Khan said: "Let's accept we left the EU, because it is clearly a fact we have… If your obsession is growth, the easiest way, the quickest way to get growth is to join the single market.
"You are still outside the EU, but by joining the single market you increase growth almost overnight.
"And by the way, what does it do to the confidence of the market? It boosts it."
Mr Khan has long pushed the idea that the UK should be a member of the single market but remain outside of the EU itself - in a similar way to Norway, Iceland and Liechtenstein.
The European Economic Area (EEA), which comprises the three non-EU countries in the single market, is subject to the same rules on the freedom of movement, goods and services as full member states follow.
A Labour source said the prospect of the UK joining the EEA had been repeatedly ruled out by Sir Keir, who has pledged to "make Brexit work" without reopening negotiations on the UK’s trade deal.
Labour has said it would negotiate a new security pact with the EU and implement various measures it says would ease tensions created by checks required by the Northern Ireland Protocol.
Mr Khan has disagreed publicly with the Labour leader on several issues, including the decriminalisation of drugs.
His comments follow Ms Truss’s speech to Conservative Party conference delegates, in which she pledged to take on the "anti-growth coalition" and "enemies of enterprise".
The London Mayor suggested that ministers cannot be serious about growth if they do not consider his policy on the single market.
"Let's not pretend that the Government is concerned about growth," he said.
"It is an ideological game they are playing, saying 'you are either with us - and by the way only our policies will lead to growth - and if you don't agree with our policies you are anti-growth'.
"It is a caricature, and it is this what we call in politics a dividing line that you try and portray, and so anyone who is not for my policies is anti-growth, anti-my policies."
On Thursday, Sir Keir said Ms Truss was the "destroyer of growth" after she accused him of being part of the "anti-growth coalition".
The Labour leader said mortgages are rising as a "direct result" of the Government's "kamikaze" mini-Budget last month.
"She's just passed a kamikaze mini-budget which has lost control of the economy, is putting hundreds of pounds on people's mortgage bills," he told BBC Radio Manchester.
"That is the absolute opposite of a plan for growth. She's... not just anti-growth, she's the destroyer of growth."