The fiancée of the leader of the failed July 21 suicide bombers was jailed for three years yesterday for helping him to escape by dressing him as a Muslim woman in a burka.

Fardosa Abdullahi was 17 when she gave Yassin Omar her mother's robes, a handbag and mobile phone the day after the attacks. She accompanied him to a North London bus station, from where he fled to Birmingham, and then repeatedly lied to police about his whereabouts.

Abdullahi, now 20, became engaged or possibly married to Omar in an Islamic ceremony four days before he tried to blow himself up at Warren Street Tube station in 2005. The Old Bailey was told that Abdullahi is mentally ill and was targeted by Omar because she was vulnerable and easily led. Five reports from psychiatrists and psychologists told the court that the Somalian woman had a long history of mental illness, including at one point hearing the voice of Omar telling her to harm herself.

Judge Paul Worsley rejected her legal team's claims that Abdullahi, who had been granted indefinite leave to remain in Britain, should be spared jail because Omar had preyed on her vulnerability.

“Your responsibility to this country should have been greater than your responsibility to your husband or fiancé, if such he was,” he said.

“The message must go out that this court will not go soft on those who assist terrorists, even those who are young, vulnerable and under pressure, as you undoubtedly were.”

Abdullahi, who wore a grey dress and headscarf, wiped away a tear as she was led to the cells. She is 24 weeks pregnant.

Her sentencing brings to an end a series of cases in which 16 people have either been convicted or admitted charges linked to the July 21 attacks. The bombs failed to detonate properly and the men fled the public transport system.

Emma Gargitter, for the prosecution, told the court that Abdullahi became engaged to Omar on July 17, despite not being at the ceremony. The day before the attacks, he used her mobile phone to send a text message to his fellow bomb plotter, Manfor Asiedu.

After the failed suicide attacks, the couple spent the night together at a hotel before she accompanied him to Golders Green bus station.

Police later released CCTV footage of Omar, who is 6ft 2in tall, dressed head to toe in a burka. He had shaved his arms in an attempt to look more feminine.

Abdullahi was contacted by Omar's family after they saw CCTV pictures of him from the Warren Street attack on television.

She telephoned police and repeatedly lied to detectives, claiming she had not been in contact with Omar since the attacks. Omar was arrested five days after the attacks. He was jailed for life with a minimum of 40 years.

Jonathan Cooper, QC, for the defence, said Abdullahi's brother introduced Omar to her as a “holy man”.

The medical reports showed that she was being treated for mental illness from a young age and had complained of hearing voices telling her to harm herself. That voice later changed to Omar's, she told doctors.

Abdullahi, from Finchley, North London, pleaded guilty on June 5 at the Old Bailey to a charge of assisting an offender.

A further count of failing to disclose information about acts of terrorism was allowed to lie on file.

The judge said that her personal circumstances and guilty plea meant that he could reduce the prison sentence from five years.

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