Mr Turner's father, Mark, wants his son and Mr McGoldrick to be released
Relatives and friends of a Dorset man who was extradited to Hungary have demonstrated outside the House of Commons and the Hungarian embassy.
Michael Turner from Corfe and his business partner Jason McGoldrick are accused of fraud following the collapse of their Budapest marketing company.
Mr Turner flew to Hungary after being extradited but has not been charged.
South Dorset, MP Jim Knight, described the use of the European arrest warrant as "inappropriate [in this case]".
Queries over protocol
He told BBC News: "The European arrest warrant has been used for a suspected offence - Mike is locked-up for 23 hours a day for just an investigation.
"He is living in appalling conditions.
"They went willingly to Hungary on the basis of agreed promises about how they would be treated that have not been kept."
Mr Knight met Mr Turner's father, Mark, and other protesters armed with placards in Westminster Hall on Monday.
Michael Turner is jointly accused of fraud in Hungary
Mark Turner handed Mr Knight personal letters for Europe Minister Baroness Kinnock and Home Secretary Alan Johnson.
He said: "The letters I have handed in ask for Jason and Mike to be immediately released and repatriated back to the UK.
"A lot of protocols have not been fulfilled - the conditions they are being kept in are far too severe."
Mr Turner also revealed that since his son flew to Hungary on 2 November, he has only spoken to him three times - all within the last week.
"My son said he feels let down by the British Government by allowing him to be extradited," he added.
Mr Turner also handed letters to Laszlo Takacs, the consul general at the Hungarian Embassy, who told BBC News: "We have asked the Hungarian authorities to provide us with the necessary information so we can answer these letters."
The men's problems began when their Budapest-based company collapsed with debts of £18,000. The Hungarian authorities allege the company's creditors are victims of fraud.
Mr Knight said he would be writing to the Justice Secretary Jack Straw about the case.