In a lengthy blog post, the lawyer responded to Mr Johnson’s recent high-profile announcement of a review of sentencing policy, investment of £2.5 billion in the construction of new prisons, and plans to strengthen stop-and-search powers.

The barrister said the additional funding for prisons and prosecutors will not reverse a decade of “chronic underfunding”.

The Crown Prosecution Service (CPS), which has been promised an additional £85 million over the next two years, has lost £250 million in real terms since 2009/10, as well as a quarter of its staff and a third of its lawyers, the barrister wrote.

However, the lawyer reserved particular ire for Mr Johnson’s “trademark opportunism and dearth of intellectual rigour”, arguing that the prime minister had “lied and lied and lied” about the impact of policing and prisons on crime rates.

The barrister wrote: “We already have the highest incarceration rate in Western Europe. Prison sentences have on average got longer year-on-year. We have more prisoners detained on indefinite and life sentences than all the other countries in the Council of Europe.

“The notion that our courts routinely hand out ‘soft sentences’ is simply not true. When we do see ‘soft justice’ stories in the headlines, they will either be an aberration, usually corrected on appeal, or they will be the product of inaccurate or dishonest reporting, removing context or omitting facts.”

They added: “He is lying so that he can turn the volume up to 11 on his remix of ‘Prison Works’ to ensure the oldies at the back of the conference hall can hear in the run-up to the inevitable autumn general election. And while Mr Johnson is lying to you, the rest of the criminal justice system rots.”