Bernie Sanders wrote to Margaret Thatcher in 1981 in a bid to put a stop to the "abuse, humiliation and degrading treatment" of IRA prisoners who were on hunger strike, documents show.
The US presidential hopeful told the former prime minister he was disturbed by the British government’s stance on the hunger strikes.
"We are deeply disturbed by your government's unwillingness to stop the abuse, humiliation and degrading treatment of the Irish prisoners now on hunger strikes in Northern Ireland, " wrote the politician, who was mayor of Burlington, Vermont, at the time.
The document was published on Thursday by the Telegraph and is reportedly part of an archive held at the University of Vermont.
Writing two months after the death of Bobby Sands, Mr Sanders said the treatment of the prisoners was damaging England's international reputation.
“We ask you to end your intransigent policy towards the prisoners before the reputation of the English people for fair play and simple decency is further damaged in the eyes of the people of Vermont and the US,” he said.
Meanwhile, recently released state documents in the National Archives in London show that Ms Thatcher was frustrated with the Irish government's lack of assistance in handing over IRA suspects to the UK.
The documents relate to the alleged delay in the extradition of Fr Patrick Ryan, an Irish priest wanted in London for his alleged connections to the Provisional IRA.
In summarising 1988 cabinet discussions on the matter, the former British prime minister said the “behaviour of the Irish authorities in the Ryan case sapped confidence in their willingness to combat terrorism in the spirit of the Anglo-Irish Agreement and made that agreement appear to many as a hollow sham”.