Former US Senator Sam Brownback reportedly told British ambassador Sir Kim Darroch that Breitbart “people” had contacted him about Robinson’s imprisonment, a government source told Reuters.
Mr Brownback reportedly told Sir Kim that British authorities should be aware that the far-right news and opinion website was making noise about the case at a meeting in June.
Reports say he suggested the UK should be more “sympathetic” to the former leader of the English Defence League, and warned Sir Kim that the Trump administration might publicly criticise its handling of the case.
Pro-Robinson campaigners claimed there were multiple direct contacts between Mr Brownback, his aides, people connected to Breitbart and other groups protesting Robinson’s imprisonment.
But a spokesperson for the US State Department said: “We refute as completely false the reports which wrongly assert that Ambassador Brownback urged the UK government to act on this issue or threatened repercussions by the US government in any way.”
Robinson was jailed for 13 months for contempt of court in May, after pleading guilty to violating reporting restrictions imposed to safeguard an ongoing set of trials at Leeds Crown Court.
A judge initially banned reporting Robinson’s imprisonment, over fears the case he commented on would collapse, but overturned the restriction after a challenge by The Independent and local media.
The 35-year-old was spared imprisonment for another contempt of court offence last year after his lawyers claimed he was ignorant of wrongdoing, with a judge telling him: “You will now be under no illusions whatsoever as to what you can and cannot do.”
Robinson, who was jailed under his real name of Stephen Yaxley-Lennon, admitted he knew about reporting restrictions in the Leeds trial during a Facebook Live video where he discussed the case and tried to film defendants.
He has a string of previous convictions including assault, mortgage fraud and travelling to the US under a false passport.
But Robinson has been forming links with the American alt-right, who characterise him as a “citizen journalist” and see imprisonment as a violation of freedom of speech.
Steve Bannon, the former Breitbart chairman who served as the White House chief strategist, has given his personal support to Robinson and former Breitbart London editor Raheem Kassam has coordinated two “Free Tommy” rallies.
Mr Bannon called Robinson a “solid guy who has got to be released from prison” on LBC radio over the weekend, before allegedly telling a journalist he was the “backbone” of Britain.
Donald Trump Jr, the US president’s son, shared a tweet from a Robinson supporter with the comment “don’t let America follow in those footsteps”, while foreign politicians including Dutch opposition leader Geert Wilders have called for his release.
A Republican senator attended a protest calling for the far-right leader to be freed from prison in London, and the event itself – which merged with a pro-Trump march – was funded by an American think tank.
The Middle East Forum, which campaigns against the supposed “Islamisation” of the US, claimed to have paid Robinson’s legal costs, organised London rallies on 9 June and 14 July, and is committed to “bringing foreign pressure on the UK government to ensure Mr Robinson’s safety and eventual release”.
Both protests saw violence break out, with his supporters filmed performing Nazi salutes and attacking police officers in June, then blockading a Muslim bus driver and attacking a trade union leader who spoke against Robinson on Saturday.
MPs and campaigners warned that far-right extremists were rallying around his imprisonment to develop a new “racist street movement” with international support.
It comes as statistics show more extreme right-wing terrorists are being arrested and jailed than ever before, with the head of MI5 warning that their brand of extremism was “rearing its ugly head” once more.
Prosecutors said Robinson’s posts radicalised Darren Osborne, the terrorist who ploughed a van into a group of Muslim worshippers in Finsbury Park last year.
Robinson is to launch an appeal against his 13-month sentence at the Court of Appeal on Wednesday.
He is only appealing the length of prison term, rather than the conviction for contempt of court as he admitted the offence himself.
In a letter purportedly written from prison last month, he said he was using the time to pen a new book.
“I’d like to thank Her Majesty for giving me the time alone on my own to work on it,” Robinson wrote.
“I sit here happy, happy that this sentence has backfired on the establishment.”
It called for a “moment to change the direction of our nation” and furthered the myth of an “Islamic takeover” in Britain and thanked supporters for donations that are supposedly going towards legal costs.
Citing support from Mr Wilders and Mr Trump, the letter continued: “I’d have done six months just for that recognition.”