The government has spent at least £600,000 of taxpayers’ money trying to prevent a civil case being brought against it by a husband and wife who allege that British intelligence was complicit in their detention, rendition and torture.

Figures released under the Freedom of Information Act reveal the extraordinary lengths to which the government is going to prevent the civil case against it, former home secretary Jack Straw, and former MI6 spy chief Sir Mark Allen coming to court.

Special report: Rendition ordeal that raises new questions about secret trials Read more

The case is being brought on behalf of Abdel Hakim Belhaj, an opponent of Colonel Muammar Gaddafi, and his wife, Fatima Boudchar, who were seized in Malaysia and rendered via Bangkok to Libya in 2004 in an operation conducted jointly by MI6, the CIA and Libyan intelligence. Boudchar says she was chained to a wall for five days and subsequently taped to a stretcher for a 17-hour flight, which left her in agony. She was held in a Libyan prison until just before the delivery of her son, who was born weighing just four pounds. On arrival in Libya, Belhaj was imprisoned for six years, during which time he says he was regularly subjected to torture.

Their claims are similar to those made by another opponent of Gaddafi, Sami al-Saadi, who alleges that he, his wife Karima and their four children, aged 12, 11, nine and six, were detained in Hong Kong before being rendered to Libya with the full knowledge and help of MI6.

Saadi’s eldest daughter, Khadija, has described hearing her mother “saying that we were being taken back to Libya to be executed by Colonel Gaddafi”, and says she “fainted, because I was sure we were going to be killed”. Saadi, like Belhaj, was held for six years and also alleges that he was tortured.

The UK’s role in the two rendition operations was revealed in 2011, when faxes from MI6 to Gaddafi’s spy chief, Moussa Koussa, were found following the fall of the regime. The faxes revealed Allen, MI6’s former head of counter-terrorism, discussing the role of UK intelligence in securing the arrival of what he termed the “air cargo”.

The Metropolitan police carried out a criminal investigation into the two renditions but the Crown Prosecution Service ruled earlier this month that there was “insufficient evidence” to bring charges.

The Saadi family received a £2m settlement two years ago. However, Belhaj and Boudchar have pursued a civil court action as they seek an apology. Supporters say the case represents one of the few remaining opportunities they have of exposing the role played by the UK government in rendition.

Facebook Twitter Pinterest Abdel Hakim Belhaj, former Libyan rebel commander. Photograph: BBC

However, information released under FOI shows that by 10 September last year the government had spent £355,000 on internal legal advice and £259,000 on external advice as it sought to have the case dropped. Of this, £27,000 was spent on advice relating to Straw and £110,000 on advice relating to Allen.

The FOI data reveals that the government has been paying as much as £250 an hour to two senior barristers involved in defending the action. A number of junior barristers have also been charging between £45 and £120 an hour.

Given that the figures are 10 months old, there is speculation that the total cost of fighting the case – before it even comes near a court – could be well in excess of £10m.

“The government has wasted over half a million pounds in taxpayers’ money arguing that torture cases shouldn’t get their day in court, simply to spare the blushes of MI6 and the CIA,” said Cori Crider, a lawyer for the victims at international human rights organisation Reprieve. “Meanwhile my clients, Abdel Hakim and Fatima, are prepared to settle for just £3 – one pound per defendant – and an apology.”

The government argues that the case should not be heard on the grounds that it would involve ruling on the activity of a foreign government. The Supreme Court is currently considering this.

The issue of rendition will be thrown into sharp relief on Wednesday when it is the subject of a parliamentary debate. “The government must use this week’s debate on renditions as a chance to finally come clean over Britain’s involvement in rendition and torture, and apologise to the victims of this shameful practice,” Crider said.

“Saying sorry to the women and children that were kidnapped and abused by British intelligence is not only the right thing to do – it will allow our country to finally move on from this dark chapter in the ‘war on terror’.”