Theresa May has attacked Shadow Home Secretary Diane Abbott over her car crash interview in which she stumbled over the cost of Labour policies live on national radio.
The police policy gaffe showed the chaos that would ensue if Jeremy Corbyn was Prime Minister, Mrs May said.
She told supporters the fact Ms Abbott gave a series of incorrect figures regarding how much Labour's plans to recruit 10,000 extra police officers will cost was a "very serious" indicator of how the party would behave in power.
Speaking at an election rally in Bristol, the Prime Minister said: "I think she was suggesting that you could employ a police officer for £8,000 a head.
"I think she needs to go and have another look at her figures.
"Actually, this is very serious, Diane Abbott wants to be Home Secretary in our country.
"I think that shows people yet again the very clear choice between the strong and stable leadership of the Conservative Party in Government and the coalition of chaos there would be under Jeremy Corbyn."
In one attempt to come up with the bill for the flagship policy, officers would earn just £30 while a second go left them with £8,000.
Ms Abbott's assessment of how many new officers would be recruited in the first 12 months of the four-year plan ranged from 25,000 to 250,000.
The shadow cabinet minister told BBC Two's Daily Politics: "I do know my figures. I did seven interviews that morning and that was the seventh and I mis-spoke but I do know my figures."
She claimed she had repeated the figures correctly during the course of six previous broadcast interviews.
"If I didn't know my figures, I wouldn't have been able to repeat them correctly in six other interviews," she told Daily Politics.
Mr Corbyn said the policy would cost £300 million, and insisted he is "not embarrassed in the slightest" by Ms Abbott's gaffe.
"She corrected the figure and that's the figure and it will be paid for by not going ahead with the cuts in capital gains tax," he told Sky News.
Asked if it was embarrassing that Ms Abbott got the figures wrong, he said: "Not at all. We have corrected the figure and it will be absolutely clear now, today and in the manifesto."
Ms Abbott repeatedly paused and stumbled as she gave a range of figures for how many extra officers would be recruited and what the bill for the plan would come to.
She initially said the four-year project would cost £300,000, then revised it up to £80 million before finally landing on £718 million.
Although the plan is to recruit 10,000 officers, Ms Abbott said 25,000 new recruits would be brought in annually, before later saying 250,000 police officers would be employed in the first year of the scheme.
She told LBC: "Well, if we recruit the 10,000 police men and women over a four-year period, we believe it will be about £300,000."
Ferrari replied: "£300,000 for 10,000 police officers? How much are you paying them?"
Ms Abbott replied: "No, I mean, sorry, they will cost, it will cost about, about £80 million."
"About £80 million? How do you get to that figure?" he said.
Ms Abbott answered: "We get to that figure because we anticipate recruiting 25,000 extra police officers a year at least over a period of four years.
"And we are looking at both what average police wages are generally, but also specifically police wages in London."
Ms Abbott went on to say that in the first year of the scheme the party expected to recruit 250,000 police officers.
"The figures are that the additional cost in year one, when we anticipate recruiting about 250,000 policemen, will be £64.3 million," she said.
When Ferrari queried the figure of 250,000 policemen, Ms Abbott responded: "And women."
Challenged again on the figure, she said: "No, we are recruiting two thousand and - perhaps - two hundred and fifty."
Ferrari asked: "So where did 250,000 come from?"
Ms Abbott responded: "I think you said that, not me."
He replied: "I can assure you you said that, because I wrote it down."
Ms Abbott then said: "What I am saying about the cost is that in year one, obviously, we are getting ready to recruit.
"But in year two, the cost will be £64.3 million. In year three, the cost will be £139.1 million. Year four the cost will be £217 million. And year five, the cost will be £298 million.
"And that can be amply covered by reversing the cuts in capital gains tax."
Starting salaries for police officers in England and Wales range from £20,000 to £23,000.