A judge jailed a disgraced MP from UK Prime Minister Boris Johnson's ruling Conservative party for 18 months on Monday for sexually assaulting a teenage boy.

The Tories expelled Imran Ahmad Khan, 48, and he resigned from his seat in parliament in the wake of his conviction after a trial last month.

He had denied plying the 15-year-old youth with alcohol and asking him to watch pornography, before assaulting him after a house party in January 2008.

At a sentencing hearing at a court in south London, judge Jeremy Baker said the former politician had shown no remorse.

"The only regret you feel is towards yourself for having found yourself in the predicament you face as a result of your actions some 14 years ago," he said.

Khan's lawyer said his client maintained his innocence and would appeal. 

A police report was made at the time of the offence but no further action was taken as the victim did not want to make a formal complaint.

But he told jurors that "it all came flooding back" when he found out Khan was standing as a candidate at the 2019 general election.

Khan's conviction and resignation has triggered a by-election in his old seat of Wakefield, in northern England.

Wakefield was one of the so-called "Red Wall" seats formerly held by the main opposition Labour party that was won by Johnson's Tories at the 2019 election.

The collapse of Labour's stronghold in northern England was key to his landslide victory. But the Conservatives are by no means certain to win the June 23 by-election.

The Tories took a hammering at recent local elections, losing some 500 seats across England, Wales and Scotland, on the back of a series of government scandals and anger at the rising cost of living.

The Sunday Times recently reported that 56 of the 650 MPs in the lower House of Commons were facing sexual misconduct claims.

One unnamed MP, also from Johnson's Conservatives, was earlier this month arrested and released on bail on suspicion of rape.