Anas Sarwar has dismissed as “a desperate lie from a desperate man” a claim by Reform UK’s Scotland leader, Malcolm Offord, that he offered to do a deal with the rightwing party to keep the Scottish National party out of power.

Offord made the claim on Channel 4’s Scottish leaders’ debate on Tuesday evening, alleging the Scottish Labour leader came “bouncing up” to him at an event in December last year, suggesting they “work together to remove the SNP”.

The row escalated when Offord told reporters after the debate that he stood by the remarks. Thomas Kerr, a Reform UK candidate in Glasgow, claimed Sarwar had made similar overtures to him some months earlier.

Sarwar immediately dismissed the accusations as “nonsense” but the SNP posted the exchange on social media, saying it was evidence of Scottish Labour seeking “a grubby deal”.

John Swinney, the SNP leader and first minister, alleges that if his party won the Holyrood elections on 7 May but fell short of an overall majority, Scottish Labour would seek Reform UK support.

Under Scottish parliament rules, a first minister has to be elected by MSPs. The convention is that the leader of the largest party is elected, but Scottish Labour and the Liberal Democrats hope the election results will mean they will collectively have enough MSPs to elect Sarwarinstead, with the support of the Conservatives.

But with Reform expected to win up to 10 seats at Holyrood, that raises the prospect Offord could hold the balance of power, potentially forcing Sarwar to accept the populist party’s support. Some Labour figures have speculated privately that could be necessary.

Senior Labour and Lib Dem sources were adamant they would never court Reform votes and said they believed Offord would abstain if such a scenario arose to avoid supporting a new Labour administration, thereby allowing Swinney to win.

Speaking to reporters after the debate, Sarwar said Offord’s claim was “desperation from a party whose campaign has completely flunked” and stated there had been “no stitch-ups, no deals, no backroom chats, no back-channel contact with Reform”.

Labour sources said Offord’s claims were a “flat-out lie”.

During the debate, Sarwar, who has faced repeated attacks about his ethnicity and loyalties from Reform, said of Offord: “One of his candidates wants to deport my children: where do you want them to go?”

Offord said: “This is the third time on national TV you’ve called me a racist. This does not square with you coming at the start of this campaign, bouncing up to me in Paisley town hall and saying we need to work together, Reform and Labour, to remove the SNP”

Speaking to reporters on the campaign trail on Wednesday, Offord insisted his account was correct. He said Sarwar had approached him in Paisley without Labour staff with him, and had said: “Interesting move to Reform. I always thought you was more of a unionist than a Tory, Malcolm.”

Offord added: “I remember that line. And he said: ‘Look, you know, you’re going todo well in the election, you’re going to win a lot of seats, at some point we need to get together and talk about how we work together, Reform and Labour, to get rid of the SNP’. Turned on his heel and left, and off he went, and that was it.”

Sarwar implied in the Channel 4 debate it was ludicrous to suggest he would offer Reform a deal given Nigel Farage’s attacks on his loyalties during the Hamilton byelection in 2025.

“These are the same people who have spent tens of thousands of pounds on racist adverts targeting me, and whose candidates have made deeply racist remarks, including calls to deport all Muslims, which would include my own children,” Sarwar said.

He also challenged the SNP’s response to Reform: “For John Swinney to jump on this when he is happy to call out Reform’s lies and racism elsewhere but somehow chooses to imply we are doing a deal with them when it concerns racism directed at me, is deeply disappointing.

“I understand why he may think it is politically useful, but morally he should take a long, hard look at himself.”

Reform has had a faltering start to the Holyrood campaign, with five candidates having stepped down or been suspended. Offord last week dismissed questions about historical offensive or Islamophobic tweets by Reform candidates, saying it was a “slippery slope” to be delving into Twitter accounts from 10 years ago.

At a rally on Monday in Aberdeen, where Reform had early success with council defections from Scottish Conservatives, Farage told a crowd of more than 300 that the 7 May elections would be “your breakthrough moment”.

The Reform leader predicted his party could become the second largest at Holyrood. “If we get the next three and a half weeks right we can, for the first time since devolution, actually have an effective voice of opposition against the SNP. [We] will become that voice of opposition,” he said.