Jonathan Aitken, the former Conservative cabinet minister who served a prison sentence for perjury, is to be ordained and plans to work as an unpaid prison chaplain.

Aitken became a Christian while serving a prison sentence for perjury in 1999 after lying on oath in a libel case against the Guardian.

Now he is to be ordained as a deacon later this month at St Paul’s cathedral by the new bishop of London, Sarah Mullally.

“I am having a slight struggle tempering my enthusiasm with proper Christian humility,” he told the Sunday Times.

The disgraced politician applied to become a deacon last year, and went through the routine process of “discernment”, during which church figures assess applicants’ commitment and aptitude. “I don’t know if they went through the press cuttings,” he said.

The diocese of London said Aitken had undergone a risk assessment and other necessary checks.

During his seven months in prison, Aitken – who was known to fellow prisoners as Jono – joined a prayer group convened by an Irish burglar. “There was a blagger, an armed robber, a blower [someone who cracks safes for a living], a kiter [a cheque forger], a couple of murderers and a dipper [a pickpocket].”

Aitken, 75, has encountered scepticism about his conversion to Christianity, but said he did not get upset about the “bucketfuls of cynicism poured over my head”.

“In a different era, I’d have been one of the cynics myself. If I’d had a parliamentary colleague who’d got into trouble, gone to jail and come out saying, ‘I’ve found God’, I’d have said, ‘Oh, how very convenient for him’.”

After his release, he studied theology but initially resisted the idea of ordination. However, the “nagging insistence” did not go away. Now, “I feel I’ve got the energy for it and, above all, I’ve got the calling for it.”

But, he added: “18 years ago, I think if they’d had any sense, the Church of England would have rejected me. Remember what a hot potato I was – going through this downward spiral of defeat, disgrace, divorce, bankruptcy and jail – so I don’t think they’d have been queuing up to have me as a curate.”

He hoped that many of his former parliamentary colleagues and ex-prisoners would attend the service on 30 June, at which 38 people will be ordained as deacons. “I live in a more real world now and am very happy with it,” he said.

Graham Tomlin, bishop of Kensington, said: “I have known Jonathan Aitken for several years and this is a calling that has grown within him and been tried and tested by many others.

“His experience of the prison system, both from the inside and the outside, gives him a unique perspective to offer Christian ministry in this vital area of our life as a society.

“He brings Christian learning from his theological studies, wide life experience, knowledge of the wider issues in criminal justice and a pastoral understanding of the needs of prisoners that will help strengthen the church’s ministry to all.”

Aitken was sentenced to 18 months at the Old Bailey after admitting perjury and perverting the course of justice.

While a government minister in charge of defence procurement, he allowed aides of the Saudi royal family to pay his £1,000 hotel bill during a stay at the Paris Ritz in September 1993.

When the Guardian obtained a copy of the bill and challenged Aitken, he told the paper that his wife, Lolicia, had paid his part of it using money he had given her. He claimed he had spent a private weekend in Paris and had not had any business meetings.

He sued the Guardian for libel, saying: “If it falls to me to start a fight to cut out the cancer of bent and twisted journalism in our country with the simple sword of truth and the trusty shield of fair play, so be it.” He repeated his claims about the hotel bill in the witness box.

The 16-day libel hearing collapsed after evidence was produced that Aitken’s wife and daughter, who was also embroiled in his account, were in Switzerland during the weekend in question when he had said they were in Paris.

Sentencing Aitken, Mr Justice Scott Baker said: “For nearly four years you wove a web of deceit in which you entangled yourself and from which there was no way out unless you were prepared to come clean and tell the truth. Unfortunately you were not.”