Giovanni Gambino (above) predicted that Cohen might face retaliation in prison

An heir to the Gambino crime family has issued a stark warning to Michael Cohen, predicting that President Donald Trump's former personal attorney may face retaliation behind bars over his Congressional testimony.

'A message for Michael Cohen: He better keep his mouth shut,' Giovanni Gambino told DailyMail.com on Wednesday morning, blasting Trump's former fixer shortly before he publicly testified.

Giovanni, the 43-year-old son of late Sicilian mob boss Francesco 'Ciccio' Gambino and cousin to infamous crime boss Carlo Gambino, speculated that Cohen's withering testimony could have harsh consequences.

'Inmates love Trump, and hate rats. If he wants to get out alive, he better keep his mouth shut about Trump,' Giovanni said.

Cohen is due to report to federal prison in May. He has been sentenced to serve three years after pleading guilty to campaign finance violations, tax fraud, bank fraud and lying to Congress.

Michael Cohen testified before a House committee on Wednesday. Mob heir Giovanni Gambino issued a stark warning, predicting Cohen might be killed behind bars as a 'rat'

Giovanni (left) is seen on a prison visit with his father Francesco 'Ciccio' Gambino before the Sicilian mob boss died behind bars in 2012

In his prepared testimony on Wednesday, Cohen directly accused his former boss of bigotry, dishonesty, and possibly criminal conduct.

'He is a racist, he is a conman and he is a cheat,' Cohen told the House Oversight and Reform Committee in stunning testimony.

Trump has blasted Cohen as crooked, dishonest and self-serving.

'He did bad things unrelated to Trump. He is lying in order to reduce his prison time,' Trump wrote in a tweet early Wednesday from Hanoi, Vietnam where he was meeting with North Korean Leader Kim Jong Un.

Trump is seen on Wednesday meeting with North Korean leader Kim Jong Un. The President accused Cohen of lying in a tweet from Hanoi

Giovanni, who has left the family business to author crime fiction, agreed with Trump's assessment.

'President Trump is right by calling Cohen a Rat,' Giovanni said. 'Cohen unnecessarily lied to the FBI and stupidly incriminated himself.'

'Unfortunately he might face the same fate as Whitey Bulger,' Giovanni added, referring to the infamous Boston crime boss turned FBI informant.

In October, Bulger was brutally killed inside a federal prison in West Virginia in a suspected mafia hit.

Cohen addressed the allegation that he is a 'rat' in his testimony on Wednesday, saying: 'I have been smeared as a rat by the President of the United States. The truth is much different ... I have always tried to live a life of loyalty, friendship, generosity and compassion.'

Giovanni is cousin to infamous crime boss Carlo Gambino (above in 1970). Carlo gave his name to the crime family and was known as the 'boss of bosses' in New York until he died in 1976

On May 6, Cohen is scheduled to report to prison to serve his 36-month sentence.

In December, US District Judge William Pauley recommended that Cohen serve his sentence at FCI Otisville, about 70 miles northwest of New York City.

The minimum-security camp at Otisville is known as one of the 'cushiest' in the federal prison system, and is often requested by Jewish inmates due to the special religious and dietary provisions offered there. Cohen is Jewish.

However, the Bureau of Prisons alone makes the final determination on where offenders serve their sentences.