A man convicted of murdering his girlfriend's mother after police overheard his prayers for forgiveness was jailed for life today.

George Maben, 45, strangled Maureen Cosgrove, 65, in her home on 24 March this year. He had been living with Cosgrove's daughter Lucy Rees, who was pregnant with his child, at the house in Carshalton Beeches, south London.

Police arrested Maben after bugging his car and overhearing his words: "God, forgive me for what I have done." He will serve at least 13 years in prison.

During the trial, the court heard that police had CCTV footage of Maben taking a bus from his mother's home to Cosgrove's residence, putting on gloves during the journey. He later met Rees, 34, and they returned to the house to find the body. Fibres from the dead woman's body were found on Maben's clothing, but police needed more evidence to charge him.

Officers placed a secret listening device in a Ford Focus used by the unemployed Maben, and on 9 April recorded his appeal for help. "Please God, help me ... for me and Lucy eliminated from all police inquiries and everything's all right, please God help me," an emotional, whispering Maben was heard to say.

"God forgive me for what I have done. I just could not take it any more. Every single day, she was breaking me down. Please God will you forgive me? Please God, sorry."

During the trial Anthony Glass QC, prosecuting, said these words should be "interpreted as a confession to murder", and said the relationship between Maben and Cosgrove had deteriorated.

"With her out of the way, the defendant could see a future for himself, Lucy, her children and the unborn baby, without the constraints imposed by Maureen Cosgrove," Glass said. "That is why he killed her and he said so in these few words. It is compelling evidence as to why he did that."

Rees told the court she had been in love with Maben, but he and her mother would sometimes argue. Rees said Maben explained the recorded confession to her by saying he was praying for forgiveness after having stolen from Cosgrove's handbag.

Maben denied murder but was convicted by a jury at the Old Bailey yesterday.

Judge Jeremy Roberts sentenced him to life imprisonment with a minimum term of 13 years. He said he was reducing the term from that which may have been expected from a premeditated murder because it was out of character and because Maben had been under pressure.

Roberts added: "It was illustrated by your prayer for forgiveness which was recorded by police".

The judge said he did not want anything he said to be interpreted as criticism of Cosgrove, who was the innocent victim of the crime. He said: "It was no one's fault that the situation developed where your relationship with Lucy, whom you loved deeply, and her mother, whom you must have seen as a obstacle, drove you to such an act of desperation."

Rees left court without commenting but her brother John told the judge in a statement: "We have lost a loving mother and grandmother in a particularly horrific way."