Scotland Yard has requested extra money from the Home Office over fears the cash will run out by the end of the month.
Some £11million has been spent on the 11-year hunt for Maddie.
Four-year-old Maddie disappeared while on holiday with her family in Portugal in May 2007.
But despite a global search, there has been no significant clues as to what could have happened.
Maddie's parents Kate and Gerry have maintained their daughter is still alive and are now fighting for funding as they fear the investigation could be wound-up in face of police cuts.
However, detectives believe they they can crack the case.
A Metropolitan Police spokesman told the Star: “The investigation continues and we are in dialogue with the Home Office over more funding.”
The Home Office said it had not yet received a formal request for further cash, but a bid would be “carefully considered”.
A source close to the family said the McCann's have "no idea" if the search will end or carry on.
They told MailOnline: "It is a daunting prospect they face once more."
"Kate and Gerry are grateful to the Metropolitan Police for everything they have done over the years and hope of course that the inquiry into their daughter's abduction will continue if more funds are requested and made available."
It comes after it was revealed detectives involved in the case have been secretly visiting Portugal in the past year.
Five return flights were booked to the country, where the toddler from Leicestershire disappeared in 2007, Met Police documents revealed.
Officers have also poured through 40,000 documents filed by Portuguese officials who have been involved in the investigation.
Maddie was three when she disappeared from a holiday apartment in the Algarve region of Portugal while her parents dined with friends nearby.
She disappeared on the evening of May 3 from her bed in a holiday apartment in Rua Dr Agostinho da Silva, Praia da Luz, where she had been sleeping with her brother and sister.
She would have turned 15 on on May 12, 2018.
A number of potential leads have emerged since the little girl vanished, but none amounted to anything and no arrests have ever been made.
In 2010, Maddie’s distraught parents met with then-home secretary Theresa May to talk about the hunt for their daughter.
The following year, Scotland Yard launched its own review, named Operation Grange, into the case at the behest of the future PM.
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