Cambridgeshire Police Authority said that if its bid was rejected, it would have to cut services or raise council taxes.

The authority has also asked the Home Office for an extra £1m to help pay for policing animal rights demonstrations against Huntingdon Life Sciences.

A spokesman from the Home Office told BBC News Online the bids had been received and would be given "very careful consideration".

He said the government hoped to "advise the police authority on the extent of the special grant as soon as possible".

Authority chairman John Reynolds said it would be difficult to meet further costs because the Cambridge force was one of the poorest funded in the country.

Holly and Jessica vanished from Soham on 4 August

"If we do not receive any assistance from the Home Office then we will have to find this whole amount from our current budget," said Mr Reynolds.

"We await the Home Office's response to our bids with eagerness and hope for a solution that provides us with the assistance we require."

The final bill is still being added up, but the authority said the murder inquiry alone had already cost £5m and costs could double.

The amount covers support staff, overtime and the bills submitted by other forces who gave their help.

Two charged

The operation involving 426 officers from 21 forces.

Former Soham caretaker Ian Huntley, 28, has been charged with the murder of the two 10-year-old girls and with conspiring to pervert the course of justice.

His girlfriend 25-year-old Maxine Carr, a learning support assistant at the girls' school, is charged with attempting to pervert the course of justice.

The bodies of the two best friends were found on 17 August in a copse near the American air force base at RAF Lakenheath in Suffolk.