Lawyers representing a Libyan husband and wife who were kidnapped and flown to one of Muammar Gaddafi’s prisons are seeking to overturn a decision that there was insufficient evidence to prosecute a former MI6 officer for his alleged role.

Mark Allen, the head of counter-terrorism at the agency at the time of the so-called rendition operations, had set out his role in a letter to the Gaddafi government that came to light during the 2011 Libyan revolution.

However the Crown Prosecution Service decided earlier this year that Allen – now Sir Mark – should face no criminal charges, a ruling that the victims said reflected poorly on British justice.

Lawyers for the couple are now seeking a judicial review of the CPS decision, which they have denounced as a “see no evil, hear no evil” ruling that has put the government and its intelligence agencies above the rule of law.

Cori Crider of the legal charity Reprieve said: “It’s sad that the CPS will aggressively prosecute journalists for ‘misconduct in public office’, but a top MI6 officer who helped render kids and a pregnant woman to Gaddafi walks free. It’s sadder still that prosecutors felt victim families should have no role in the ‘victims’ review.’ The family’s view is clear. Justice was denied. The decision leaves the UK’s security services basically exempt from British law. That’s why they are fighting it.”

Abdel Hakim Belhaj, an opponent of Gaddafi, and his wife Fatima Bouchar, were abducted in Thailand in March 2004 while en route from Malaysia to the UK, where they had planned to apply for political asylum.

They were held briefly at an American detention facility in Bangkok, and allegedly tortured, before being bound and gagged and flown to Tripoli. Bouchar, who was pregnant, says she was taped head to foot to a stretcher for the 17-hour flight.

Evidence of MI6 involvement emerged in correspondence between Allen and Gaddafi’s intelligence chief, Moussa Koussa, that was discovered during the Libyan revolution.

Other documents included a fax from the CIA to Tripoli, which gave details of a second UK-Libyan rendition operation, in which another opponent of Gaddafi, Sami al-Saadi, was kidnapped in Hong Kong and flown to Tripoli along with his wife and four children.

Scotland Yard spent four years investigating the kidnappings and sent a 28,000-page file to the CPS. Police are understood to believe that they had built a particularly formidable case against MI6 in the Saadi case.

Detectives are said to have been dismayed that the CPS concluded there was insufficient evidence to bring charges against Allen or anyone else. In making public their decision, the CPS disclosed that Allen had “sought political authority for some of his actions”.

In a letter to the CPS, Belhaj’s lawyers at the London firm Leigh Day claim that the decision is unreasonable. They say: “It appears the CPS has erroneously proceeded on the basis that political approval would be a reasonable excuse for the conduct under investigation.”

Meanwhile, the legal charity Reprieve has complained to the information commissioner over the government’s refusal to disclose whether ministers or Downing Street officials were in any way involved in the decision to not bring charges.

There was no comment from the CPS or Allen, who was not directly named in the CPS statement.