London Mayor Sadiq Khan will in a speech Thursday urge the government to end a "vow of silence" on the "immense damage" to the UK economy caused by Brexit.
Khan, an opposition Labour politician, will say that "trying to will Brexit into a success, or simply ignoring its impact, is not a strategy that will deliver prosperity for London or a brighter future for Britain".
Excerpts of the speech to London's political and business leaders were released to media.
Two years after Britain's departure from the European Union, it is widely acknowledged that UK businesses are reeling from the cost of Brexit, including some who voted to cut ties with Brussels.
"We can't -- in all good conscience -- pretend that it isn't hurting our people and harming our businesses," Khan will say.
He will also say the estimated cost to the Treasury in lost tax revenues owing to Brexit is £40 billion ($49 billion).
"As mayor... choosing not to say anything would be a dereliction of duty."
He will add: "I was elected on a pledge to stand up for all Londoners and for our business community. That's why I simply can't keep quiet about the immense damage Brexit is doing."
The Conservative government led by Prime Minister Rishi Sunak has said it believes the UK economy has entered a recession on fallout from sky-high inflation.
While it repeatedly blames this on Russia's invasion of Ukraine fuelling energy prices, analysts claim Brexit has also pushed up costs.
Bank of England monetary policy committee member Swati Dhingra recently told MPs that Brexit was to blame for "a much bigger slowdown in trade in the UK compared to the rest of the world".
Khan's speech will say that "ministers seem to have developed selective amnesia when it comes to one of the root causes of our problems.
"Brexit can't be airbrushed out of history or the consequences wished away," he will add.