The 63-year-old was pictured in The Sun on a date with a blonde woman in her 20s in the town of Sumy, 1,500 miles from his home in Hartlepool.
He was freed early on licence in January 2011 after being sentenced in 2008 to serve six years and three months for fraud.
That meant he was not allowed to leave the UK without Probation Service permission until all of his sentence was served.
A source close to the case said: "He is facing a return to prison for travelling abroad without permission."
The Probation Service would not speak about individual cases but a spokesman said: "Any offender subject to licence supervision is required to gain permission from probation to travel outside of the UK; permission is only granted in exceptional circumstances.
"Any offender who travels without this permission will be subject to recall to custody.
"In these circumstances the Probation Service works closely with the police to implement the recall."
According to The Sun, Darwin and his date, a local woman named Anna, enjoyed a two-hour meal assisted by a translator, but the evening turned sour when he was confronted by a reporter.
The newspaper said Darwin first made contact with the woman over the internet.
He faked his own death in a canoeing accident in 2002 so his then wife Anne could claim hundreds of thousands of pounds from insurance policies and pension schemes.
The couple, from Seaton Carew, were jailed at Teesside Crown Court in 2008 for the swindle, which deceived the police, a coroner, financial institutions and even their sons Mark and Anthony.
Darwin admitted fraud so received a slightly shorter sentence than Anne, who denied the offences. They have now divorced.
After faking his own death, Darwin continued to live in secret with his wife before they escaped to Panama to start a new life.
But in December 2007 Darwin walked into a London police station claiming he had amnesia and was reunited with his stunned sons.
His wife, then still in Panama, initially also claimed to be surprised - until a photograph emerged of them posing together.