Significant numbers of jobs are being put at risk across the UK because the Government is ignoring service industries in Brexit talks, London's mayor will warn.
Focusing on trade of goods at the expense of sectors like financial services, accountancy and the legal profession could seriously damage the economy, according to Sadiq Khan.
Appearing before MPs, he will claim Prime Minister Theresa May "has her priorities all wrong" in the negotiations over Britain's exit from the European Union.
The Labour mayor will tell the Public Administration and Constitutional Affairs Committee that trade in services accounts for around 40% of UK exports to the EU.
Around 92% of London's economy is made up of the service sector, the same as Manchester, while it accounts for 91% in Edinburgh, 83% in Leeds and 82% in Birmingham.
Ahead of the committee meeting, Mr Khan said: "The United Kingdom has a vibrant services industry responsible for employing more than 20 million people across the country.
"Any deal struck by the Government that focuses just on goods, at the expense of services, could seriously damage our economy.
"The Government's current approach is further evidence, as if we even needed it, that the Prime Minister has her priorities all wrong.
"London is unquestionably the best place in the world to do business. The city is built on strong foundations of innovation, entrepreneurship, creativity and a world-class financial hub - and that is not going to change.
"But we are already seeing major banks establishing subsidiaries in other EU countries, or moving part of their business out of the capital because EU law requires them to be legally compliant from the day the UK leaves the European Union.
"If the Government does not change its approach and strike a deal that secures access to the single market for services, this trend will only continue.
"The result will be fewer jobs, less investment and less prosperity the length and breadth of the country."