The probe into the alleged airliner bomb plot is costing the Metropolitan Police £1m a week, commissioner Sir Ian Blair has said.
Seventeen people have now been charged over the alleged plot
The funding of anti-terror operations was leaving the force in "very considerable difficulties", he said.
Sir Ian told the Metropolitan Police Authority such costs could not be absorbed within their current budget unless they stopped other police work.
Seventeen people have been charged as a result of Operation Overt.
Sir Ian said the Met, between 9 August and 22 September had had to spend £5.2m on overtime and £3.3m on support costs - mutual aid from other forces, travel, accommodation and property services.
This amount, £8.5m, was over and above the £9.7m the Met spent during that period paying the wages of the officers involved.
The commissioner said: "We're saying - you know, we've got people sitting in custody - you have got to give us the forensic, and you've got to give it to us now, I mean that's priority charging for the forensic science service, and the costs are extremely high.
"The job of myself and of the authority is to explain to government that there is now a new cost element which we just can't absorb within current budgets unless we stop doing some other things that the people of London and other places want us to do."