In July, Basel-headquartered announced that the Swiss Federal Tax Administration had asked it for information on current and former France-based clients, based on data from 2006 and 2008. UBS said at the time that the legal grounds for the disclosure order “are ambiguous at best” and “lack the required specificity”. It revealed that the order stems from information France received last year from German authorities, who as part of their own tax-related probes seized data relating to UBS clients booked in Switzerland.

On Monday, Le Parisien published extracts from a letter from French tax authorities dated 11 May. It said France was chasing the names of the owners of “more than 45,161 different account numbers”.

“The assets of those listed totalled more than SFr11 billion ($11 billion), which could represent a several billion dollar shortfall for the French treasury,” according to the letter, the report said.

French authorities have reportedly so far identified 4,782 accounts and are seeking to find the owners of a further 40,379 accounts.

The bank declined to comment to WealthBriefing on the matter, saying it had nothing to add to a statement made in June this year, when it said: "UBS has received a disclosure order from the Swiss Federal Tax Administration (FTA) to transfer information based on a French request for international administrative assistance in tax matters. The request concerns a number of UBS account numbers pertaining to current and former French domiciled clients and is based on data from 2006 and 2008. Since then, the client base underlying the data has changed significantly and a large number of the accounts affected by the French request are closed."

The statement continued: "The French tax authorities filed the request for international administrative assistance in tax matters with the FTA based on the Double Taxation Agreement between Switzerland and France. The FTA has accepted the request and directed a disclosure order to UBS.

"Pursuant to the disclosure order, UBS is required to produce the requested information to the FTA. The bank has expressed its concerns to the FTA that the legal grounds for this request are ambiguous at best. This includes the view that the data and the justification received as part of the request lack the required specificity. UBS has taken steps to inform affected clients about the administrative assistance proceeding and their procedural rights, including the right to appeal. To ensure legal clarity UBS also plans to take legal steps to have the admissibility of the administrative assistance request evaluated by the Swiss Federal Administrative Court," it added.

"The request from the French tax authorities is based on data received from the German authorities. As previously reported, German authorities conducted various investigations on tax matters in recent years. Certain data related to UBS clients booked in Switzerland was seized during these investigations and also apparently shared with other European countries. UBS expects other countries to file similar requests.

"The information seized also includes data from 2009 mainly related to Swiss-domiciled private clients. UBS has taken steps to inform these clients. They represent a very limited population of the overall client base of the bank in Switzerland. A large part of the data refers to mortgage and pension accounts. The Swiss-domiciled private clients are not part of the French request.

"UBS has largely completed a compliance programme with clients based in Europe, including France. The bank was among the first in the industry to take this step requiring documentation of tax disclosure by its clients. The request should also be seen in light of the fact that as per 1 January 2017, the automatic exchange of tax-relevant client data between states (AEI) will take effect in Switzerland, under which all Swiss banks will have to provide data to French and other tax authorities on an annual basis," it added.