A Labour MP accused of lying to police about a speeding offence behaved like a politician when she demonstrated the “skill of not giving a straight answer” and being “flexible with the truth”, a court has heard.

Fiona Onasanya, 35, allegedly weaved a “succession of lies over many months” after her Nissan Micra was caught doing 41mph in a 30mph zone in Thorney, Cambridgeshire.

The Peterborough MP has said she does not know who was driving her car when it was caught speeding shortly after 10pm on 24 July last year. She denies perverting the course of justice.

A notice of intended prosecution (NIP) was sent back to authorities claiming her former lodger was behind the wheel. Police established he was in Russia with his parents at the time.

Days before the trial began, the MP’s brother Festus admitted three charges of perverting the course of justice.

In his closing speech, David Jeremy QC, prosecuting, said the MP had built a “dishonest contrivance” by “taking a scalpel” to the evidence and cutting out the bits that were uncomfortable for her.

He said Onasanya responded to questions in a way “sometimes attributed to politicians”.

Jeremy told jurors: “I doubt that you will derive a great deal of pleasure from having to decide this case. It should of course have never come to this: a trial at the Old Bailey.

“Ms Onasanya is undoubtedly a talented, hard-working public servant. You should make every allowance for everything that you have heard in her favour.”

However, he said, even after making allowances for her busy life as a new MP, Onasanya must take responsibility for avoiding the questions from the police and telling “lie after lie”.

“What she has done, she alone is responsible for – starting with the very bad choice she made when she received that notice of intended prosecution,” he said.

Jeremy said many people might consider pretending someone else had been driving their car if it was caught speeding and they received that “annoying piece of paper through the post”. He said: “It is one thing to think about it and another thing to do it.”

He said Onasanya’s alleged lie was “not just one aberration” and she could have changed her case at any time. “It was a succession of lies over many months,” he said.

Onasanya has told jurors she had assumed she was in Westminster at the time of the speeding offence and left the NIP at her mother’s house in Cambridge for whoever had borrowed her Nissan Micra to fill in. She told the jury that Festus had told her the NIP had been “sorted out”.

The defence lawyer Christine Agnew QC, in her closing speech, heaped blame on to Festus, describing him as a “charming chancer”. When the pair went together to be interviewed by police, he was banking on her status as a solicitor and MP to get him “out of the hole”, jurors were told.

Agnew said: “He is a dishonest chancer. He is somebody who will manipulate anyone to get out of a hole. If he was driving, that is exactly the motivation that would be required to fill that form out wrong.”

She suggested Festus filled in the form as he had done it before and “thought he could get away with it”. He already had nine points on his licence and so risked disqualification and the subsequent loss of his job as a delivery driver, she said.

Agnew said Onasanya was not a woman who “lies, lies and lies again” as the prosecution claimed.

“What’s she guilty of? She’s guilty of not completing the NIP properly. She’s guilty of not asking her brother as many questions as she should have done. You cannot choose your family, ladies and gentlemen. That’s a trite comment but she had no reason to suspect him and she had no reason to disbelieve him. Why would she put everything at risk for three penalty points?”

Nick Brown, the shadow chief whip, said in a statement read to jurors by the defence: “Fiona is a decent, outgoing character. She is bright, able and committed to her job.”

The trial continues.