Liz Truss could face two separate probes over claims she didn't declare thousands of pounds spent on a champagne dinner attended by Tory MPs.
Labour has written to the Cabinet Secretary calling for an investigation into the 'Fizz with Liz' event at a private members' club in Mayfair, London, in October.
It is claimed the event, reportedly paid for by Conservative donor Robin Birley, cost £3,000.
Her campaign team has said the wannabe PM did nothing wrong - saying Ms Truss did not organise the event and did not benefit from it.
Meanwhile the Lib Dems have written to the Parliamentary Commissioner for Standards, Kathryn Stone, demanding a probe into the event.
Labour deputy leader Angela Rayner said: “ Liz Truss is corked from the same vintage as Boris Johnson.
"She thinks the rules don’t apply to her and she can take the British public for fools.
"There must be an urgent investigation into the murky donations made by this millionaire aristocrat and why as Foreign Secretary, she has failed to come clean about them."
Ms Rayner and shadow Commons leader Thangam Debbonaire have also written to Cabinet Secretary Simon Case asking him to investigate the matter.
MPs are required to declare any gifts, benefit or hospitality worth more than £300 received from a UK source.
Lib Dem deputy leader Daisy Cooper said: "The last thing this country needs is another Boris Johnson. Yet here we are again, this time hearing allegations of rule-breaking levelled at the Conservatives' frontrunner for prime minister. You'd be forgiven for feeling a sense of deja vu."
Ms Cooper added: "Liz Truss must come clean and explain who paid for what at her schmoozing events. I hope the Parliamentary Commissioner for Standards will get to the bottom of it before public trust is damaged any further."
In a letter to Cabinet Secretary Simon Case, Ms Rayner and Ms Debbonaire wrote: "The large sum of hospitality funding gifted for this event has not been declared either on the Foreign Secretary's ministerial gifts declaration, October to December 2021, or in her Register of Members' Financial Interests, as at 1 November 2021.
"There are questions about whether the Foreign Secretary was acting in her ministerial capacity, or her capacity as a Member of Parliament - however she failed in her duty on both counts.
"Indeed, by using parliamentary paper and explicitly inviting colleagues within her capacity as an MP via her parliamentary email address, Ms Truss's event clearly falls under the transparency requirements of the Members' Code of Conduct.
"However, as Foreign Secretary, it is likely Conservative members attended this event in order to be in the company of the Foreign Secretary, and it would therefore be expected that she would make ministerial declarations in the usual way.
"There are serious questions for the Foreign Secretary to answer about why she failed to declare this large sum of hospitality funding, and why she now claims she had 'nothing to do' with the event's organisation, despite sending out personal invitations to all attendees."
They added: "We ask that you do the necessary investigation into the numerous questions raised by these allegations. The public has a right to know why such a significant donation for hospitality was not properly declared by the Foreign Secretary."
A spokesperson from Ms Truss' campaign told The Mirror: “A declaration would only need to made if there was a benefit to Liz, or it was above the threshold for individual declarations for MPs register of interests."
There were a number of MPs there, and the event was not organised by Ms Truss, her team said.
