UPDATE 1-France sets 1 bln eur bail as it probes HSBC over Swiss tax scandal

(Changes headline to clarify HSBC is under investigation, not charged)

LONDON/PARIS, April 9 (Reuters) - French magistrates have ordered HSBC Holdings Plc (HKSE: 0005.HK - news) to post a 1 billion euro ($1.1 billion) bail to cover a potential fine after the bank was put under formal criminal investigation over allegations it helped customers dodge tax.

It is the latest problem to hit HSBC over a tax evasion scandal at its Swiss private bank. HSBC said it intends to appeal the French magistrates' decision, which it said was "without legal basis and the bail is unwarranted and excessive".

The amount of the deposit is almost half of the scale of the alleged fraud by the HSBC, a person familiar with the matter said. The magistrates estimate HSBC's fraud is about 2.2 billion euros, the source said.

The source said HSBC contests that figure, and the bank has always disputed the number of clients the French authorities have claimed its Swiss bank had.

The source said the deposit had to be posted by June 20, which would allow an appeals court to consider HSBC's appeal.

HSBC is the second big bank to have a hefty bail imposed on it by French magistrates in the last year, after Switzerland's UBS (NYSEArca: FBGX - news) had to provide 1.1 billion euro to the court to cover a potential fine for alleged money laundering. UBS's appeal against that bond was dismissed.

Under French court process, companies can be ordered to post a deposit when they are put under formal investigation, known as 'mise en examen', even if charges are not brought.

The latest move is against HSBC's holding company, relating to matters in 2006 and 2007, and steps up the pressure on the British bank after a series of scandals in recent years have prompted criticism it is too big and complex.

HSBC has admitted failings in controls at its Swiss private bank following media reports that it helped wealthy customers conceal millions of dollars of assets.

HSBC's Swiss Private Bank had already been placed under formal criminal examination by magistrates in both France and Belgium, but the latest move is against the holding company. HSBC's Swiss unit previously posted a 50 million euro bond with French magistrates.

Various tax, regulatory and law authorities, including in Britain, Argentina and Switzerland, are also looking at the allegations.

($1 = 0.9289 euros) (Reporting by Steve Slater in London and Chine Labbe in Paris; Editing by Sinead Cruise)