Sudan is facing mounting condemnation for sentencing a pregnant woman to be whipped and then hanged for adultery and apostasy, and for keeping her shackled in prison with her toddler son a month before she is due to give birth.

Governments, the UN and human rights groups have called on the Sudanese government to immediately release Meriam Yahya Ibrahim, 27, and overturn both her death sentence and sentence of 100 lashes. More than 100,000 people have backed a call by Amnesty International to release Ibrahim.

Ibrahim was arrested after a Muslim relative claimed her marriage to a US citizen was invalid, and thus adulterous, because he is a Christian. Ibrahim was also found guilty of apostasy. But she said she had been brought up a Christian and refused to renounce her faith.

Her lawyers have lodged an appeal against the sentence, which may be heard in Khartoum this week. Ibrahim is being held in harsh conditions and is constantly shackled, according to Amnesty. Her 20-month-old son, Martin, has been kept in prison with her since February.

Ibrahim has been told that her execution will be deferred for two years to allow her to deliver and then wean her baby.

Her husband, Daniel Wani, who left Sudan for the US in 1998, has travelled to Khartoum to try to secure the release of his wife and son. He said Ibrahim was being denied medical treatment and he had not been allowed to visit her or Martin, according to media reports.

The Sudanese authorities have reportedly refused to release the child to his father's care because of his Christian faith.

Ibrahim – a graduate of Sudan University's school of medicine – told the court she was the daughter of a Sudanese Muslim father and an Ethiopian Christian mother, but was raised as a Christian after her father left the family when she was six.

According to Human Rights Watch, article 126 of Sudan's criminal code says a Muslim who renounces Islam is guilty of apostasy, punishable by death, unless the accused recants within three days.

The UK government has summoned Sudan's chargé d'affaires in London to the Foreign Office to hear its "deep concern".

In a statement, Foreign Office minister Mark Simmonds said: "This barbaric sentence highlights the stark divide between the practices of the Sudanese courts and the country's international human rights obligations." The Sudanese government must respect the right to freedom of religion or belief, he added.

US senators Kelly Ayotte and Roy Blunt have raised the case with the secretary of state, John Kerry, calling for "immediate action and full diplomatic engagement to offer Meriam political asylum and secure her and her son's safe release".

The department's spokeswoman,, Jen Psaki, said on Wednesday the US was "deeply disturbed" by the case and called on Khartoum to respect the right to freedom of religion. The Canadian and Dutch governments have also expressed concern.

The UN has also urged Sudan to adhere to international law. "We are concerned about the physical and mental wellbeing of Ms Ibrahim, who is in her eighth month of pregnancy, and also of her 20-month-old son, who is detained with her at the Omdurman women's prison near Khartoum, reportedly in harsh conditions," said Rupert Colville of the UN Human Rights Office in Geneva.

Amnesty said: "The fact that a woman has been sentenced to death for her religious choice, and to flogging for being married to a man of an allegedly different religion, is appalling and abhorrent."

Wani, who has a biochemical engineering degree and suffers from muscular dystrophy, lives in Manchester, New Hampshire and became a US citizen in 2005. He and Ibrahim met in Khartoum, and were married there in 2012. Wani had taken steps to bring Ibrahim to join him in the US.

Last year, a relative accused Ibrahim of adultery, saying her marriage to a Christian was invalid. The authorities later added the charge of apostasy.Gabriel Wani, Daniel's brother, who also lives in Manchester, New Hampshire, said Ibrahim was in poor physical shape. "Meriam is in a bad condition, she is eight months pregnant. She needs proper medical attention and she needs medical supplies. She's bleeding and nothing is being done," he told the Daily Mail.

"She needs to eat well but she is just getting the prison food. When she had her first son it was a very difficult birth, she lost a lot of blood. She is supposed to have check ups with the doctor but it isn't happening. We are praying for a miracle."