Officer numbers in the Metropolitan police will fall to their lowest level since 2002 unless hundreds of millions of pounds in new funding can be secured, London’s Labour mayor has said.

A new forecast by the mayor’s office for policing and crime (Mopac), which is responsible for law enforcement in the capital, says the Met still has to find £325m in saving because of government cuts. The force currently has just under 29,700 officers.

The forecast projects that by 2022, further planned cuts will take Met officer numbers down to 26,800, for a population of more than 9 million people. When Met officer numbers were last that low in 2002, London’s population was 7 million.

The forecast does not take into account any increase in funding that may be announced next week when the government announces a financial settlement for policing.

Leaks put out by the Conservatives suggest local government will be allowed to raise council tax further to fund the police and there may be some extra national government funding. Even then, informal estimates suggest that would leave the capital with around 27,500 officers, assuming the leaks are correct.

A decision by the Treasury meaning police forces have to pay more toward pensions for their officers will cost the Met over £100m a year, again hitting officer numbers.

Rising crime and the consequent rising public concern has led to the Conservatives dropping their insistence that the police budget could be cut.

Since 2010, when the Conservatives came to power nationally, Mopac says London has lost 3,000 police officers, another 3,000 police community support officers and 5,000 civilian staff. The Met has made £850m in savings, including selling its headquarters and moving into a smaller building.

The new Mopac forecast assumes any savings would be made purely by cuts to officer numbers, and paying and maintaining staff is the biggest cost the Met faces.

London’s mayor, Sadiq Khan, said: “The causes of violent crime are extremely complex, but there is no doubt it has been made far worse by huge government cuts to the police and youth services.

“Even the home secretary has finally admitted that the Met won’t be able to tackle violent crime without more funding from the government.

“Now we urgently need to see action to avoid officer numbers falling even further. I’m genuinely concerned about how we keep Londoners safe with officer numbers as low as 26,800.

“Ministers need to reverse the £1bn savings forced on the Met and reverse their cuts on youth services and other preventative services so that we can keep our city safe.”

Khan will meet the home secretary, Sajid Javid, on Tuesday to discuss police funding, with the announcement from the Treasury about any extra money for the police expected on Thursday.

London’s population has been growing since the start of this century and is forecast to hit 10 million by 2030.

Until 2010 police numbers were broadly stable at around the 30,000 mark or just above.

In 2010 it had 33,367 officers for a population of 8,054,000, meaning it had 4.1 police officers per thousand people.

Now it has 29,654 officers for a population of 9 million, or 3.3 officers per thousand people, and the number of officers is forecast to fall, despite a rising population and hundreds of thousands of visitors to the capital every year.

Historically, London has had more officers per hundred thousand population than other big urban forces because the capital is seen as having more complex needs.

Police chiefs nationally are threatening to sue the government unless it relieves the new financial burden placed on forces by the government’s decision on police pensions.