Labour insiders have expressed concern about Tory plans to ramp up attacks on Sir Keir Starmer’s tenure as director of public prosecutions, Sky News can reveal.

The Labour leader has repeatedly referenced his time leading the Crown Prosecution Service between 2008 and 2013 as a core part of his political pitch to voters.

At PMQs earlier this month, he boasted: “I have prosecuted thousands upon thousands of sex offenders. The prime minister has just shown that he does not understand how the criminal justice system works. No wonder he cannot fix it.”

But some Labour Party insiders have reservations about this approach, with one telling Sky News: “If your record involves as many controversies as Keir Starmer, it’s probably not great political strategy to draw attention to it.”

Staffers in the attack unit at Conservative Campaign Headquarters (CCHQ) have been combing through cases for more than a year.

Senior Tory sources say they believe Sir Keir’s past is a vulnerability they can exploit, having identified a number of examples they think will change the public’s view of the Labour leader.

Red Knight, the unauthorised biography of Sir Keir by Tory pollster Lord Ashcroft, has emerged as what some in CCHQ see as an attack bible – chronicling controversial cases, including the prosecution of journalists for phone hacking.

“There’s a lot of material out there,” a senior Conservative source explains.

“One of the areas where Starmer is really open to criticism is this constant claiming that he personally prosecuted this person or that person, and then when something went wrong and someone wasn’t prosecuted, or the case was messed up, he says ‘It’s nothing to do with me’.

“You can’t have it both ways – the person at the top of the organisation is responsible and is the one who will have to issue a public apology when things go wrong.”

Sky News can reveal that one such case that the Tories will use to attack the Labour leader in the coming months is the so-called “Twitter joke trial” of 2010.

The controversial case hit the headlines when Paul Chambers from South Yorkshire was found guilty at Doncaster Magistrates Court for sending a “menacing” tweet about wanting to blow Robin Hood Airport “sky high” because it was closed due to snow.

Mr Chambers said he did not think his “silly joke”, which he sent in January 2010, would be taken seriously – but he was nevertheless arrested and charged under the Communications Act, for sending messages of a “grossly offensive, indecent, obscene or menacing character”.

Mr Chambers subsequently appealed his case and won, with a number of high-profile comedians including Stephen Fry, Al Murray and Graham Linehan all backing him.

Labour sources expressed concern that the case could play into the caricature the Tories have painted of Sir Keir as “Sir Softie” – the “lefty lawyer” they hope to portray as out of touch and too politically correct.

The Tories insist their main focus is establishing their own “competence of government” following a series of scandals and leadership dramas.

But with the local elections just around the corner and the general election fast approaching, “you can expect this sort of stuff to come out”, the Conservative source says.

The first public attack on Sir Keir’s past record came when former prime minister Boris Johnson accused the Labour leader of failing to prosecute the former television presenter and prolific sex offender Jimmy Savile.

The Labour leader hit back strongly against the claim, which he perceived as a slur, and pointed out that he was not the lawyer responsible for reviewing the case.

However, as head of the DPP, others argued that he bore the ultimate responsibility.

It is a viewpoint some believe Labour has endorsed itself with its recent attack ads against the prime minister – the most controversial of which suggested Mr Sunak did not think child sexual abusers should go to prison.

Some Labour sources fear the personal nature of the criticism against Mr Sunak – as well as the suggestion that he personally is responsible for the past 13 years – could “legitimise” any further Tory attacks on Sir Keir’s record as DPP.

One Labour insider said the ad had “deflected attention from Labour’s positive message and what the Tories have done to the country and onto trivia and Starmer’s record.”

They added: “If your record involves as many controversies as Keir Starmer, it’s probably not great political strategy to draw attention to it. In fact, it’s more like a case study of what not to do.”

Another party source said: “One of the problems with the ad is that it makes Keir’s record as DPP fair game. I’m sure the Tories were going to attack it anyway, but now we’ve just invited it on.”

But one shadow cabinet member who spoke to Sky News dismissed the idea the ads have spurred on the Tories’ efforts to attack Sir Keir.

“The Tories will always get down and dirty in their campaign because that’s what they do.

“They can hit us as hard as they like, but Keir’s record as DPP is really strong – he’s locked up thousands of dangerous criminals and terrorists – while they literally broke the law in Downing Street.

“If they open up this as an attack, our defence will be stronger than their attack. You’re either up for the fight or you’re not – but we are and we intend to win it.

“Bring it on.”

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A Labour spokesperson said Sir Keir would continue to point to his experience as a reason voters should place their trust in him:

“As the country’s most senior prosecutor, Keir Starmer got criminals off the streets, locked up terrorists and prosecuted MPs who cheated their expenses.

“At the heart of his approach were victims; their rights, their confidence in the system and securing them justice. Keir was a reforming director of public prosecutions, acting where he saw failure and driving through change when it was needed. He is rightly proud of his record.”