A British man held in Myanmar’s most notorious jail is wasting away in a tiny cell with no window and an open sewer for a toilet while the British government is ignoring pleas to intervene, his father has told the Guardian.

Philip Blackwood, 33, and two Burmese colleagues were accused of insulting religion and sentenced to two-and-a-half years in jail in March after he posted a psychedelic image of Buddha wearing headphones on the Facebook page of the Yangon bar he manages.

“Philip has loss 50 pounds (20kg) since he was arrested,” Brian Blackwood, from Middlesbrough, told the Guardian from the New Zealand capital Wellington where he lives with his wife Angela.

When they last visited Philip in June, they found “awful conditions. There was a hole in a corner as a long drop into a open sewer. He was sleeping on a pallet and there was a window but it had been blocked off”.

“He put on a brave face. He doesn’t want us to see him suffering,” he said. Philip was fed “rice and gruel” and was now getting some protein once or twice a week. “For the first three months, Philip was deprived of any contact from outside the prison,” he said.

The top security Insein jail is notorious worldwide for torture and inhumane conditions, mainly for political prisoners. Former inmates say the prison is tantamount to a death sentence and the fortress is known locally as the “darkest hell-hole in Burma”.

Philip spends 14 hours a day in the airless cell, which is only 1.8 x 2.4m (6ft by 8ft) and shares common space with 11 other non-Burmese prisoners from around Asia.

Blackwood’s fiancée, a Filipina, can visit every two weeks and has managed to deliver two blankets, some canned food and the sedative Xanax after her husband’s mental condition faltered.

“He’s finding it difficult to cope with the situation. Some of the inmates are predators, they are child rapists. They come out with horrible things they have done to the children,” Brian Blackwood said. “Philip wanted something to calm him down.”

Philip’s daughter was two months old when he was incarcerated. The first time he saw his lawyer was in court. “He spent 36 days in further imprisonment without seeing people from the outside. He didn’t know what was going on.”

Philip Blackwood, who also holds a New Zealand passport, denied he had intended to offend Buddhism. After he realised people were angered by the post, he swiftly removed the picture and wrote an apology.

His father said the case came during a period of growing Buddhist nationalism in Myanmar and was being used by the military government in the run up to elections.

The family are waiting for an appeal to scheduled but are not confident.

“The appeal means nothing because the prison sentence was due to outside influence – we’re talking about the generals and the hardline monks. With the elections coming up next month, they want to show that they won’t stand for this.”

“We’ve been in contact with the British government. We are getting very little response from them,” Blackwood said. “We desperately need some political intervention here.”

An electrician married to a retired nurse, Blackwood said that Middlesbrough MP Andy McDonald has “done a really good job of asking questions in parliament”.

The family has also been working with the Burma Campaign UK charity which accused foreign office minister Hugo Swire this week of abandoning Blackwood’s case by refusing to call for his release during a recent visit to Yangon.

Mark Farmaner, director of Burma Campaign UK, said: “Hugo Swire seems prepared to let an innocent British citizen and his colleagues rot in a Burmese jail rather than risk upsetting his new friends in the Burmese regime.”

A UK foreign office spokesman told the Guardian that the British ambassador to Myanmar, also known as Burma, had raised the case directly with the president’s office.

“It is misleading and unhelpful for the Burma Campaign to suggest that we do not raise difficult issues with the Burmese government. Hugo Swire set out our concerns very clearly on a range of human rights issues during his visit in July, and again at the UN last week.”

The spokesman said the New Zealand embassy “is leading on providing assistance to Mr Blackwood and his family as he travelled to Burma on a New Zealand passport.”

Brian said the government in New Zealand is too “concerned about their trade ties with Myanmar” to exert political pressure.

The Blackwoods moved to New Zealand when Philip was four and his sister Jenny was three.