LONDON (Reuters) - Britain's finance minister Rachel Reeves is expected to use a major speech this week to "promote free and open trade between nations" amid the threat of potential tariffs under Donald Trump's presidency, The Observer newspaper reported on Saturday.

Trump, who won the 2024 U.S. presidential election, has proposed imposing a 10% tariff on imports from all countries, and 60% duties on imports from China.

In her first major speech to the financial and professional services industry on Thursday, known as the 'Mansion House Speech', Reeves is "expected to be clear that she will take the fight to Washington in defence of free trade", The Observer said.

Reeves said on Wednesday she would make "strong representations" over the importance of free trade to Trump's incoming administration.

A leading British think tank, the National Institute of Economic and Social Research, said the country's already slow economic growth rate could be more than halved if Trump imposes tariffs.

Feeble British growth would be a setback for British Prime Minister Keir Starmer, who has promised voters he will turn the economy into the fastest-growing in the Group of Seven.

The annual 'Mansion House Speech' is often used to sketch out future plans for the industry and is closely watched for clues on the government's next steps on issues like regulation.

(Reporting by Catarina Demony; Editing by Chizu Nomiyama)