Labour did nothing wrong when party officials campaigned for Kamala Harris in the US election, a former Conservative minister has argued, after Downing Street faced fury from Donald Trump about the move.

Robert Buckland, who has also campaigned for Harris due to his distaste for Trump, said it appeared that Labour activists who knocked on doors had volunteered and covered their own expenses, which would not be a breach of US laws on overseas involvement in elections.

Trump’s campaign filed a legal complaint alleging that apparent efforts by Labour’s head of operations to organise volunteers amounted to “illegal foreign national contributions”, and hit out at what it called Keir Starmer’s “far-left” party.

After Starmer said he believed the row would not affect his relationship with Trump, Labour officials insisted that the party had no role in organising or funding staff who joined US campaigning efforts, and that such volunteering was by no means unusual.

The Trump legal letter, sent to the US Federal Election Commission in Washington, also complained about what it called “strategic meetings” at August’s Democratic national convention in Chicago between Harris’s team and Morgan McSweeney, now the prime minister’s chief of staff, and Matthew Doyle, Starmer’s communications director.

Labour officials said that the pair were at the event only as observers. The party paid for McSweeney to attend, and Doyle’s costs were covered by the Progressive Policy Institute thinktank.

Buckland, a former justice secretary, who lost his Commons seat at the general election, said a since deleted LinkedIn post by Labour’s head of operations offering to arrange housing for 100 current and former party officials campaigning for the Democrats in swing states was “unfortunate”.

However, he told the Guardian he did not see any sign of wrongdoing. “It doesn’t look like it to me,” he said. “If these individuals are going under their own steam, paying for their own flights and doing their own thing, and their accommodation is either they’re staying with friends or they’re paying for it, there’s not a problem. But they’ve played into the Trump-Vance campaign hands, and that press release was the sort of politicking that you’re going to see this close to an election.”

Starmer, speaking to reporters travelling with him to the Commonwealth summit in Samoa, said such volunteering had happened at “pretty much every [US] election”. He said: “They’re doing it in their spare time, they’re doing it as volunteers, they’re staying I think with other volunteers over there.”

Asked if it risked jeopardising his relationship with Trump if he becomes president again, Starmer said: “No. I spent time in New York with President Trump, had dinner with him, and my purpose in doing that was to make sure that between the two of us we established a good relationship, which we did.”

There was some muted criticism of the government from the Conservatives, although Oliver Dowden, the party’s deputy leader, did not raise it with Angela Rayner when she filled in for the absent Starmer at prime minister’s questions.

John Lamont, the shadow Scotland secretary, told BBC Radio 4 that Labour had created a “diplomatic car crash” that risked undermining relations with Trump.

Nigel Farage, the Reform UK leader, told GB News that the LinkedIn post seemed to show a “very clear breach of American electoral law” and he did not believe the Labour staffers had covered their own costs.

Farage attended the Republican national convention in Milwaukee in July. His entry in the MPs’ register of interests says the near £33,000 costs for him and a staffer were paid for by a Thai-based British businessman, Christopher Harborne. Farage listed the purpose of the trip as “to support a friend who was almost killed and to represent Clacton [his constituency] on the world stage”.

The former prime minister Liz Truss also attended the event, although by then she was no longer an MP.

One Labour MP, Ruth Cadbury, used a holiday in September to campaign for Harris in New Hampshire, while no sitting Conservatives are known to have volunteered in the same way. Almost none have publicly endorsed Trump.

Buckland said this did not surprise him, calling Trump “not a Republican”. He said: “I think most Conservatives would identify themselves with Ronald Reagan and Dwight Eisenhower and George HW Bush, and even George W Bush, not this character.”

This article was amended on 24 October 2024. An earlier version said that Robert Buckland “stepped down” as an MP at the 2024 general election. In fact, he lost his seat.