* Cameron requested Met investigation into Madeleine's disappearance.
* Police fear funding for 'Operation Grange' could be cut just 15 months after it was launched

The Metropolitan Police fears funding for its investigation into the disappearance of Madeleine McCann could be cut. 

Scotland Yard has asked David Cameron for a new commitment to financing the inquiry, which he requested 15 months ago, as the search for the missing youngster continues to draw a blank. 

If the funding is discontinued the investigation will be forced to wind down.

Disappearance: Madeleine McCann went missing from her family's holiday apartment in Praia da Luz, Portugal

Madeleine, who was six at the time of her disappearance, was last seen in her parents' holiday apartment in Praia da Luz, Portugal, in 2007.  

Operation Grange was opened by the Met last year and involves 28  detectives.

It is believed to have cost around £2.5m so far.  

Detectives said earlier this year that there was a chance Madeleine could still be alive.

They said they were pursuing dozens of leads and firmly believed she was abducted. 

Portuguese police, however, have refused to re-open the case.

Search: Madeleine's parents Kate (left) and Gerry McCann have not given up hope of finding their daughter

Mr Hogan-Howe said the police are sifting through a 'large cache' of documents.  

He said 'there will be a point at which we and the Government will want to make a decision about what the likely outcome is,' according to the London Evening Standard.

'The Prime Minister agreed to fund this.  

'We have not had any pressure to say you must stop spending more than x amount, we have received a lot of support. 

'But by Christmas I would want a very clear view of what work is outstanding and what time line there is for that.' 

Madeleine's parents Kate and Gerry have been desperately searching for her since May 2007.