The bank worker who leaked HSBC Swiss bank account details has claimed he first contacted HM Revenue and Customs in 2008 and authorities have only a “tiny part” of the information available.
Herve Falciani, who initially obtained the list while employed as an IT worker in 2007, told Sky News he had emailed and phoned HMRC two years before data was handed over by French authorities in 2010.
His comments come amid an escalating political row about the disclosures.
Mr Falciani, who told the broadcaster he felt offshore banking was “out of control”, said: “I sent an email, a very naive email, in 2008 ... to England - to the department dedicated to tax evasion - and afterwards I even called them.
“And finally the most efficient move was through the French authorities because when we accepted to work together it was established and agreed that what we were doing should be available to any any countries having co-operation treaties signed with France.”
He added more “could have been done before” and there was “100-fold” more information than UK tax authorities currently have available.
“If you put (the data) together, if you get the investigators joining the effort and the information then you will have a huge case, but right now, I repeat, the British investigators receive just a tiny part,” he said.
In the Commons yesterday, HMRC chief executive Lin Homer insisted that staff had been “diligent” in their approach to the files, and had secured one conviction and £135m in unpaid tax, fines and interest.
Earlier, Labour deputy leader Harriet Harman indicated Ed Miliband is prepared to repeat allegations that former Tory party treasurer Stanley Fink engaged in “tax avoidance activities” without the cloak of parliamentary privilege.
Lord Fink has said the comments by the Labour leader in the Commons during Prime Minister’s Questions were “untrue and defamatory”.