The lawyer of Christopher Tappin, who was jailed in the US for arms dealing, said the British businessman had to endure a prison that was "teeming with rats and run by gangs".
Karen Todner, who appeared to confirm his return to the UK, told the BBC:
For six weeks while his repatriation was being approved, he was moved to the Metropolitan Correctional Centre in New York, a prison which is teeming with rats and run by gangs.
Mr Tappin is an upstanding, proud man and he told me he just tried to stay in the background there.
After that, his hands and feet were shackled while he was taken to JFK Airport to be flown home. His family are relieved he is back, particularly as he suffers chest problems.
A British businessman who was sent to prison in the US for arms dealing has reportedly returned to the UK to serve the remainder of his sentence.
Christopher Tappin was extradited last year and pleaded guilty to one count of aiding and abetting the illegal export of defence articles.
He was sentenced to 33 months in prison and fined $11,357 (£7,095) in January after a so-called plea bargain with US prosecutors.
US District Judge David Briones recommended that Tappin, from Orpington, Kent, should be allowed to serve his sentence in the UK.
Christopher Tappin has been repatriated to the UK, his lawyer Karen Todner has announced on Twitter.
In January, the businessman was given a 33-month prison sentence for his role in an arms export plot after striking a deal with prosecutors in the US.
Tappin, 66, from Orpinton in Kent, admitted to one count of aiding and abetting the illegal export of defence articles in a plea bargain to reduce his sentence.
British businessman Christopher Tappin has been jailed for 33 months for his role in exporting batteries that could be used in missiles.Read the full story ›
US Attorney Robert Pitman gave the following statement regarding Christopher Tappin's sentencing today:
In this case, Mr. Tappin admitted his role in trying to facilitate the shipment of specialized batteries to Iran.
These batteries are used to make Hark missiles operational, and Mr. Tappan admitted that he submitted false shipping documentation to circumvent US export control regulations.
Those who violate federal law for monetary gain, and in the process put the national security of the United States and its allies at risk, will face prosecution and punishment for their callous disregard for the public's safety.
Elaine Tappin - Christopher's wife of 38 years - has said she is praying that her husband will be allowed to serve his sentence at a UK prison.
Now I can begin to see light at the end of this long dark tunnel - but remain frustrated that Chris' extradition was granted in the first place.
Being returned to a US prison will be dreadful for him. He is learning to live with the regrets - it is a chastening experience after a 45-year unblemished business career ...
I pray that the US and UK authorities between them will expedite Chris' repatriation to Britain - so we, and his friends, are able to visit him in prison.
Elaine, who suffers from chronic illness Churg-Strauss Syndrome, was unable to attend the court in Texas.
Christopher Tappin's wife, Elaine, said she hoped her husband would have "the mental fortitude to cope with whatever lies before him in the months and years to come".
US District Judge David Briones recommended that Mr Tappin should be allowed to serve his sentence in the UK but the decision ultimately rests with the US Bureau of Prisons, a spokeswoman for the court added.
He will commence his sentence at the Allenwood prison in Pennsylvania and must turn himself in to start the term by March 8.
Christopher Tappin was fined $11,357.14 (£7,095) in addition to his 33-month prison sentence for selling surface-to-air missile parts to Iran, a spokeswoman for the US District Court in El Paso, Texas said.