An NGO has called for an investigation into five far-right members of the European parliament, warning of a potential failure to declare expenses for a trip to attend a gala dinner in New York headlined by Donald Trump.

Transparency International’s EU office has written to the parliament’s watchdog on MEP conduct requesting an inquiry into five politicians over a potential failure to declare travel and tickets to the black-tie gala hosted by the New York Young Republican Club (NYYRC) in December 2023.

According to the NGO, two of the MEPs may also have failed to declare expenses for attending the NYYRC gala in 2022, and one may have breached the rules over a discussion panel organised by the same group in New York.

The complaint concerns two Hungarian lawmakers who are key members of the country’s ruling Fidesz party, Kinga Gál and Ernő Schaller-Baross; the Alternative für Deutschland MEP Maximilian Krah, who stood down as his party’s lead candidate after saying the SS were “not all criminals”; a former mayor in Italy’s far-right League party, Susanna Ceccardi; and a leading figure in Austria’s anti-migrant, anti-Islam Freedom party, Harald Vilimsky.

In responses to the Guardian, four of the five lawmakers – Gál, Schaller-Baross, Krah and Ceccardi – rejected any suggestion of breaking rules and said variously that their travel had been funded by their government, political party, European parliament group, or their own funds, which is in line with the rules. Vilimsky did not respond to questions.

The complaint highlights the links between Trump’s Maga universe and Europe’s far right, who share hostility towards liberal rules-based institutions, climate action, and refugees and migrants.

Two of the MEPs, Ceccardi and Vilimsky, are listed as members of the NYYRC. It is understood this does not breach European parliament rules.

All five MEPs were photographed at the NYYRC gala in 2023, where delegates paid between $699 and $1,399 to be addressed by Trump over a “multi-course French-service” dinner with an open bar featuring “top-shelf liquor”.

In a rambling speech over 80 minutes, Trump told diners he wanted to be “a dictator for one day because I want a wall, and I want to drill, drill, drill”. Other guests of honour included the far-right former Trump adviser Steve Bannon, who was jailed for defying multiple congressional subpoenas related to the 6 January attack on the US Capitol; the Trump acolyte Rudy Giuliani, who was ordered to pay $148.1m in damages for lies about the 2020 US presidential elections; and the extremist commentator and conspiracy theorist Jack Posobiec.

Gál, a veteran MEP who co-leads Viktor Orbán’s far-right Patriots for Europe group in the European parliament, also attracted scrutiny from Transparency International over who paid for her to attend an NYYRC panel event called Viktor Orbán – Defender of Europe on 15 November 2023 in New York.

Transparency International, an NGO dedicated to integrity in public life, called on the European parliament’s advisory committee, which vets potential breaches of the code of conduct, to investigate how the MEPs funded their attendance, “to exclude any wrongdoing”.

Shari Hinds, a policy officer at Transparency International, said: “For us it’s very important that there should be complete and full transparency regarding the funding source [of any travel].”

The group made the appeal to the advisory committee after it became aware that the MEPs had attended the 2023 gala and other events but failed to find any reference to how their travel was funded in their public declarations.

Under the European parliament’s code of conduct, which was tightened up in 2023 after the Qatargate scandal, MEPs are obliged to declare when a third party pays for them to attend an event, including travel, accommodation and other expenses. There are exemptions for attending events funded by a long list of third parties, such as EU national and regional governments and European political parties. Nor are MEPs expected to declare trips made at their own expense.

Gál said the costs of her attendance at both events was paid for by Fidesz and her participation was “in no way related” to her MEP duties.

Schaller-Baross said he was invited and attended the 2023 gala as the Hungarian government’s ministerial commissioner for foreign relations at the prime minister’s office. He said his travel, accommodation and related expenses “were covered by the prime minister’s office”.

Ceccardi said her attendance at the gala was paid for by the far-right Identity and Democracy group, a European parliament group superseded by the Patriots alliance.

The MEP, a member of the parliament’s US relations delegation, said her trip to the US from 8 to 10 December 2023 was covered by the European parliament’s additional travel allowance, which allows MEPs to claim for reimbursement of foreign travel that is not part of official parliament meetings. The allowance is worth €5,500 a year.

Transparency International said it had made a freedom of information request to the parliament that found “Susanna Ceccardi had not used the additional travel allowance in the period from October to December 2023”.

Asked about the discrepancy, Ceccardi’s office said it stood by its earlier statement, while Transparency International said: “We were clear in our request that we were seeking documents for travel that took place during those dates, not documents submitted on those dates.”

Asked about the discrepancy, a European parliament spokesperson said: “The EP cannot not comment on individual cases and the ongoing work of the advisory committee, as it is strictly confidential.”

Ceccardi’s office said: “Any insinuation that MEP Ceccardi received financial contributions or other benefits from third parties is factually incorrect. Such allegations are defamatory and risk causing significant damage to her reputation and personal integrity, making them prosecutable in the appropriate judicial offices.”

Krah’s office said he had paid to attend the NYRRC gala, plus travel expenses, for two years running “out of his own pocket”. A source close to Krah said one of his assistants had to lend him cash to pay for his gala ticket in 2023 because he had left his credit card in Strasbourg. The money was subsequently repaid, it is understood.

The complaints have been sent to the European parliament’s advisory committee, a cross-party group of eight MEPs that assesses rule breaking and recommends action. The committee is expected to meet in the coming days. It works in secret and has no fixed timetable for addressing complaints.