(Reuters) - A campaign group has begun a legal process that could lead to Britain's tax authority facing a judicial review over its failure to take a tougher line with people alleged to have used accounts with an HSBC unit in Switzerland to evade tax.

The February leak of documents on thousands of Swiss bank accounts has caused embarrassment for business people and politicians across the world and left HSBC facing scrutiny from lawmakers and law enforcers in Europe and the Americas.

Rights group Avaaz said in a statement on Friday that it had written to Her Majesty's Revenue & Customs (HMRC) asking why it offered Britons named in the leaked records the opportunity to escape prosecution by bringing money onshore via a tax amnesty.

If Avaaz is unhappy with the response, it can apply to the High Court for a judicial review on whether HMRC's actions were lawful.

Avaaz is represented by law firm Leigh Day, which represented campaign group UK Uncut in its action for a judicial review of a tax deal HMRC cut with Goldman Sachs.

A High Court judge ruled in that case that HMRC had acted lawfully, but the revelation of details of the negotiations between the two sides was embarrassing for both.

HMRC said it had acted "effectively" on the HSBC data and "will strongly contest any action".

HSBC declined to comment.