The party said the event, described in reports as a “champagne dinner” in a Mayfair members club, may have breached spending rules for MPs.
The Independent, which first reported the story, said club owner Robin Birley paid for the event which cost around £3,000.
MPs must declare hospitality over £300, but a source in her campaign said she did not organise the event and only attended as one of a number of Tory MPs.
Parliamentary rules mean MPs must declare “benefits which relate in any way to their membership of the House or political activities, if provided by a UK source either free or at concessionary rates”.
But a spokesperson for her campaign said: “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.”
Labour said Ms Truss had “serious questions” to answer over the event, and asked the Cabinet Secretary Simon Case to investigate the matter.
In a letter reported by the BBC, the party’s deputy leader Angela Rayer and Shadow Commons leader Thangam Debbonaire said the event did not appear among Ms Truss’s declarations.
The letter states there are “questions about whether the foreign secretary was acting in her ministerial capacity, or her capacity as a member of Parliament” at the event.
It adds: “The public has a right to know why such a significant donation for hospitality was not properly declared by the foreign secretary”.
The 5 Hertford St club is popular with Conserative party donors and is reported to be among the most exclusive private members clubs in the capital.