"You add all those things together, and you get a dramatic change in budgets of a scale that's never going to be absorbed by efficiencies, and is going to require some pretty eye-watering cuts to sort of to the services we provide to London."
He added that he was not going to get into detail at this stage on the "tough choices" the force would face without an increase in resources.
But he said he planned to specific "10 or 20 things we're going to do differently" before Christmas.
He added that the implications for policing in the capital would "become more public" in the coming weeks.
The budget for the Home Office is set to shrink by 3.3% next year in real terms, with the bulk of this coming from assumed savings on asylum support.
At last month's Budget, the government said it planned to "increase the core government grant for police forces," although it did not specify whether this would be in cash terms or taking account of inflation.
Before the election, Labour also said it also planned to save £360m through more efficient purchasing of police equipment, which it promised to spend on extra community support officers.