He also refused to support calls for a target of “zero murders” to be set in London, saying this was not “realistic” and he did not want to give “false hope”.
The number of Met police officers has increased by 874 to 32,475 since Mr Khan took office in May 2016, according to the latest data.
The net increase has been partly driven by London receiving extra officers following a 2019 government pledge to provide an additional 20,000 across the country over three years.
Met Commissioner Dame Cressida Dick and Mr Khan believe the capital is entitled to 6,000 of the new recruits but only received 1,369 last year, with a further 1,370 due this year, following £255million of Home Office funds.
Mr Khan, asked how many police would be in post by 2024 if he won a second term, told the Standard: “I want to be honest with Londoners. It depends how successful we are negotiating the third year of the 20,000 officer deal. The bad news is that the Government has only given us around 1,300 in the first two years.”
But he “guaranteed” that funding was in place from City Hall to ensure the extra officers that had been recruited to date could be retained for four years.
Separate to the Home Office funding, Mr Khan has directly funded 1,300 additional officers — 300 by increasing his share of council tax and 1,000 from diverting business rate income.
His main rival, Tory Shaun Bailey, has vowed to increase the Met’s strength to 40,000 officers, though his plan also relies heavily on Home Office support.
Crime has risen to the top of Londoners’ concerns in the mayoral race, according to recent polls for the Standard and ITV London.
The first TV debate involving the four main candidates was last night.
The ITV Savanta ComRes poll found 43 per cent of respondents believed Mr Khan was doing “badly” in dealing with knife crime and gangs, compared with 30 per cent saying he was doing well.
But respondents said Mr Khan was the candidate most likely to keep them and their families safe, with 33 per cent preferring him against 23 per cent for Mr Bailey.
Sian Berry, the Green candidate, has proposed a 10-year target to reduce the number of murders to zero. There have been 126 in the last 12 months.
She said a zero murders target would lead to a sea-change in prevention.
“This is a public health approach to crime,” she told the Standard. “No murders are random. They are not the weather. There is always a cause.”
But Mr Khan said: “I think it’s important to be ambitious but I’m keen to be realistic. I’m also keen not to give people false hope. We are a global city. On an average day we have more than 10 million people in London.
“Our comparator isn’t Manchester or Liverpool or Birmingham, it’s New York. I can’t hand on heart say that during the foreseeable future we will have zero murders in London. What I can say is that we will continue to build on the progress in the first term.
“Road deaths happen in the public realm. Some murders happen in the home – an awful type of homicide is domestic violence homicide, where the police have never been called before.”