More than 80,000 people have signed a petition calling for the release of a British grandfather who they fear could die from 350 lashes he is due to receive for making his own wine.
Karl Andree, who has survived cancer and suffers from asthma, was arrested in Jeddah, Saudi Arabia in August last year for breaching the country's strict anti-alcohol laws.
The grandfather-of-seven, who has lived in the Middle East for 25 years, has served his time in jail but is still locked up as Saudi officials wait to carry out the lashings, according to his son Simon Andree.
Other reports suggest Saudi officials never intended for the 74-year-old to be flogged, due to his age and health, but his family have questioned the delay in his release from prison.
Mr Andree's son Hugh told The Sun (£) the public outcry sparked by his father's situation showed "what's important to the man on the street".
News of the Prime Minister's intervention, after weeks of pleas by the family to the Foreign Office, was welcomed by Mr Andree's daughter Kirsten Piroth.
She said: "It is great news. The only reason for doing this (going public) was because we were hoping the Government would get involved. We just wanted something to happen and if it takes the Prime Minister to write a letter then that's great."
Ms Piroth said she did not want to point fingers but had been left feeling "pretty helpless" by the Foreign Office.
She said: "We got lots of nice emails and we started to feel like it was going nowhere. Our dad was in prison, he's been pretty ill. I'm really surprised that he's lasted so long."
A spokesman for the Foreign Office said: "Minister (Tobias) Ellwood, our ambassador and other senior embassy staff have raised our deep concern over Mr Andree's case with the Saudi government.
"We are actively seeking his release as soon as possible and without the corporal punishment taking place. We will continue to press the government to resolve this urgently."
Mr Andree's family are also urging his release on compassionate grounds because his wife Verity is dying of Alzheimer's and is in Britain receiving care.
In a separate development, the British government withdrew a the prison contract bid with Saudi authorities after reports of a Cabinet rift on the issue, with Justice Secretary Michael Gove said to have angered Foreign Secretary Philip Hammond by seeking to pull the plug.
Mr Gove was reported by The Times to have secured the support of Business Secretary Sajid Javid for abandoning the proposed deal to sell expertise to the Saudi penal system - but was overruled by Downing Street.
The Justice Secretary has closed down the controversial departmental commercial body which sold prison expertise to other countries, some with poor human rights records.
Just Solutions international (JSi) was established under his predecessor Chris Grayling in 2013.
Although JSi was disbanded, the bid for the Saudi work remained on the table.
Labour leader Jeremy Corbyn claimed the Government had been "shamed into a U-turn" on the prison contract.
"We should be sending a strong message to repressive regimes that the UK is a beacon for human rights and that this contract bid is unacceptable in the 21st century, and would damage Britain's standing in the world," he said.