The Shadow Chancellor has said Theresa May will have "questions to answer" if allegations related to the offshore affairs of one of the Conservative party's biggest donors prove to be true.
John McDonnell was responding to an expose published as part of the 'Paradise Papers' investigation, by The Guardian, the BBC, Suddeutsche Zeitung and other media outlets, detailing the financial dealings of the rich and powerful.
They have analysed around 13.4 million files said to have been leaked, about 18 months after the disclosure of the Panama Papers sent shockwaves through the world of business.
The leaked documents suggest Lord Ashcroft, who has donated millions to the Tories, had a previously unknown offshore trust which at one point was worth over £300m.
Reports based on the Paradise Papers by the BBC claim the former Conservative Party Treasurer retained non-domiciled tax status in Belize at a time when it was generally thought he had given up the status.
Mr McDonnell has said the Prime Minister should be prepared to answer questions given her party's pledge to bring transparency to the offshore industry.
"If the identification of Lord Ashcroft, a major Tory party funder, on the list and if the allegations of tax avoidance are true, it means that the Prime Minister has questions to answer," the Shadow Chancellor said.
"What did she and the Conservative Party know about Ashcroft's tax affairs and what due diligence checks were applied before she agreed to the Conservative Party accepting significant donations from him?" Mr McDonnell added.
Lord Ashcroft's spokesman, Alan Kilkenny, told The Guardian the peer had never engaged in tax evasion, abusive tax avoidance or tax avoidance using artificial structures, and that "any suggestion or implication that he has will be vigorously challenged".
The Panama Papers resulted in lasting damage for a number of the figures named, including Iceland's Prime Minister, who stepped downafter suggestions he held investments in Iceland's collapsed banks, and former Pakistan PM Nawaz Sharif.
Liberal Democrat leader Sir Vince Cable called for MPs to investigate the claims about the so-called Paradise Papers.
"The Paradise Papers suggest that a small number of wealthy individuals have been able, entirely legally, to put their money beyond the reach of the Exchequer," said Mr Cable.
"Given these revelations, including news that Conservative donors benefited from these arrangements, we need a parliamentary select committee to investigate fully who decided what and why," he added.
"In particular, we need the release of all government papers dealing with the decision not to clamp down on offshore tax havens. Only in this way can we ensure there is full public confidence in the tax system," said the Lib Dem leader.
Labour leader Jeremy Corbyn added on Twitter that the disclosure "proves" that "there's one rule for the super-rich and another for the rest when it comes to paying tax."
Responding to the latest revelations, a government spokeswoman said: "Since 2010, the Government has secured an additional £160 billion, more than the annual UK NHS budget, for our vital public services by tackling tax avoidance, evasion and non-compliance.
"This includes more than £2.8 billion from those trying to hide money abroad to avoid paying what they owe. There are 26,000 HMRC staff tackling tax avoidance and evasion, and we have provided an extra £800 million to fund their efforts"
"A fair tax system is a critical and key part of our plan to build a fairer society, and we are clear that everyone must pay what is due, at the right time."