LONDON (AFP) - Britain's finance watchdog said on Thursday that it has given banks until next week to reveal whether they have any links to the law firm at the centre of the Panama Papers leak.

A spokesman for the Financial Conduct Authority (FCA) said it had written to about 20 companies asking to explain their relationship with Panama-based Mossack Fonseca by April 15.

The British deadline is for the completion of initial investigations into Mossack Fonseca or companies managed or formed by the firm, according to the Financial Times, which published extracts from the letter.

"Beyond 15 April, we will require updates on any significant issues or relationships identified and a full response, detailing your findings, when your investigation is concluded," the letter read.

French banks have also been asked for "additional" information on their activities in tax havens, a spokesman for France's banking and insurance watchdog ACPR told AFP.

French newspaper Le Monde reported earlier this week that the trio of British banking giant HSBC and Swiss institutions UBS and Credit Suisse set up more than 4,500 offshore companies through Mossack Fonseca.

HSBC created 2,300 offshore companies, Credit Suisse has 1,105 and UBS has 1,100, while France's Societe Generale has 979, according to the latest revelations from the vast leak of documents dubbed the Panama Papers.

Le Monde reported that 365 banks across the world had used the services of Mossack Fonseca, the firm at the centre of the allegations.

Other major banks mentioned in the report include Germany's Deutsche Bank and Nordea, which does business in Nordic and Baltic countries.

Offshore companies can be used for legitimate purposes, but they have in the past been used to launder money and evade taxes.

The banks cited by Le Monde have denied any wrongdoing.