Sir Bernard Hogan-Howe told the BBC's Andrew Marr "you can only make so many efficiency savings" before front-line services are affected.
Sir Bernard, who retires at the end of the month, said public service spending had been hit since 2008.
A Home Office Spokesman said: "The Metropolitan Police Service remains the best funded force in the country."
But the country's most senior officer said: "If you look across the whole of the UK the number of cops has come from 147,000 to 127,000.
"The likelihood is by 2020 there will be even less public spending, and there's another Â£3bn [of savings] to find."
The Home Office said police funding has been protected since the 2015 spending review.
"The public should be in no doubt that the Metropolitan Police will continue to have the resources its needs to cut crime and keep our communities safe," a spokesman said.
The outgoing Met Police Commissioner has warned against further cuts to police budgets and officer numbers.