A teenager has been jailed for at least 16 years after being found guilty of murdering a high-ranking civil servant after they met via the gay social networking app Grindr.

A judge said Ben Bamford, then 17, caused “merciless carnage” by using at least three knives to inflict more than 40 injuries on Paul Jefferies, 52, including slashing his throat.

Jefferies, a senior HM Revenue and Customs official who reportedly advised ex-chancellor George Osborne’s Treasury team, was found naked with a tea towel over his head on his kitchen floor in Mayfield, East Sussex, in February.

Paul Jefferies. Photograph: Sussex Police/PA

Bamford had set out to get money from Jefferies on 23 February because he was under pressure to pay drugs debts of about £400, a jury heard during a two-week trial at Lewes crown court.

Bamford, now 18, denied murder, claiming he was protecting himself from Jefferies, who had allegedly “come on to him”. Jurors convicted the teenager on Tuesday after three hours of deliberation.

After leaving Jefferies on the kitchen floor, Bamford took Jefferies’ car and drove to Eastbourne District general hospital, where he was admitted after claiming he had self-harmed, to avoid police involvement. Jurors were shown a selfie picture of Bamford flicking his middle finger while lying on his hospital bed.

Sentencing Bamford, of Crowborough, Mr Justice Spencer said he murdered Jefferies in a drug-fuelled “outburst of extreme violence”, possibly over money or sex.

The home of Paul Jefferies in Mayfield, East Sussex. Photograph: Gareth Fuller/PA

The judge told Bamford: “In total you inflicted in excess of 40 knife wounds to the head and body of Paul Jefferies, with at least three knives.

“They included stab wounds to his face, close to the right eye and to his forehead, where the knife scored the skull.”

He added: “The overall attack must have lasted several minutes, during which time he was conscious and mobile.

“You must have realised by the end that he was fatally injured. You placed a tea towel over his head and shoulder as he lay dying on the floor, probably because you could not bear to look at what you had done.”

The judge said Bamford’s account of what happened revealed only “a fraction of the truth”.

He continued: “The reality is that you must have stabbed him many times in the bedroom, in an outburst of extreme violence, as the photographs clearly demonstrate.

“Precisely what triggered that outburst we shall never know unless and until you choose to disclose it. It may have been to do with sex. It may have been to do with money.”

Mr Justice Spencer told the court Jefferies said and did nothing to “excuse or explain” the killing.