The first in a series of controversial Labour attack ads on Rishi Sunak over the Conservatives’ record on crime has now been seen more than 21 million times.The party’s first advert, which alleges the prime minister “doesn’t believe” adults convicted of sexually assaulting children should go to prison, has generated days of headlines since it was first posted on Twitter on Thursday.

The claim is linked to Labour research suggesting 4,500 individuals with such convictions have not served prison time under the Conservatives since 2010.It has sparked significant criticism across the political spectrum, with some Labour figures joining voices from other parties in condemning it. The Conservative party’s deputy chair Lee Anderson called it “vile”.

But former Home Secretary David Blunkett and former Shadow Chancellor John McDonnell both also said the Labour party was “better than this”.Twitter itself has also added a notice to the tweet, saying current sentencing guidelines allow for jail terms of up to 14 years and there are no Conservative plan to change them. Critics point out Sunak himself only become an MP in 2015, whereas Labour leader Keir Starmer was director of public prosecutions until 2013.

But Labour has doubled down on the advert and some senior figures have defended it, as the party gears up for the local elections in early May.Labour has released further similar attack ads since which claim Sunak does not believe in punishing thieves or jailing criminals who carry guns with intent to harm. It is also reported to be planning another on the Conservatives’ record on rape.

The Sunday Times claimed some Labour strategists were “cock-a-hoop” with the response to the advert.The initial advert over the Conservatives’ record on sexual assaults on children had been seen more than 21 million times on Twitter by mid-afternoon on Sunday. The post on gun crime had 3.8 million views, and the one on punishing thieves 1.2 million.

By contrast, a graphic earlier in the week on rebuilding neighbourhood policing received under 300,000 views, and a video featuring Starmer promising to make Britain’s streets safe has seen fewer than 150,000 views. Labour’s “antisocial behaviour action plan” graphic was seen 176,000 times. It comes only a week after the Home Secretary Suella Braverman singled out Labour-run councils’ record on grooming gangs for criticism.