Police investigating Madeleine McCann's disappearance say a critical line of inquiry is still being pursued, 10 years after she went missing.
Met Police Assistant Commissioner Mark Rowley said they still hoped they could provide answers to what happened.
Three-year-old Madeleine disappeared from a holiday apartment in Praia da Luz, Portugal on 3 May, 2007.
Her mother Kate described the approaching 10th anniversary as a "horrible marker of time, stolen time".
Asked if police were any closer to solving the case than they were six years ago, Mr Rowley said: "I know we have a significant line of inquiry which is worth pursuing, and because it's worth pursuing it could provide an answer, but until we've gone through it I won't know whether we are going to get there or not."
But he added that there was no "definitive evidence" as to whether Madeleine was alive or dead.
He declined to expand on the nature of the working theories or reveal whether any suspects were currently being considered but said the investigating team were still receiving evidence and new information from members of the public on a daily basis.
He said UK and Portuguese detectives were doing "a critical piece of work and we don't want to spoil it by putting titbits of information out publicly".
But he added that "there was no reason whatsoever" to suspect Madeleine's parents were involved in her disappearance.
"However she left that apartment, she's been abducted," he said. "This is a young girl who is missing."
He said: "We've got some thoughts on what we think the most likely explanations might be and we are pursuing those."
Four detectives are working on the case in the UK, scaled back from 30 in 2011, with more than £11m spent so far.
Scotland Yard started its review of the case in May 2011, after then-Prime Minister David Cameron responded to a plea from the McCanns.
Tom Symonds, BBC home affairs correspondent
The Madeleine McCann investigation, codenamed Operation Grange within the Metropolitan Police, has always suffered from the inadequacies of the original Portuguese investigation.
Just one was the fact that mobile phone call records from Praia Da Luz that night were collected, but not interrogated for clues.
At least they were collected, says the Met, which has now caught up with the huge task of examining them. But who knows what opportunities have been missed?
Detectives on the case have developed hypotheses about Madeleine's disappearance and it is clear they are still trying to confirm at least one of them - a "significant", "critical" line of inquiry.
Even if this bears fruit, it may not answer that chilling, decade-old question: Where is Madeleine McCann?
And Operation Grange, with its unusual government funding, can't go on for ever. No more leads can only mean no more money - and the end.
Ahead of the 10th anniversary Kate McCann wrote in a message on the Official Find Madeleine Campaign Facebook page: "Ten years - there's no easy way to say it, describe it, accept it.
"I remember when Madeleine first disappeared I couldn't even begin to consider anything in terms of years...
"And now here we are... Madeleine, our Madeleine - ten years."
She said she and husband Gerry were "bracing ourselves for the next couple of weeks" of coverage.
With "no new appeals that the police wish to make at this moment in time" she said the family, from Rothley, Leicestershire, was "keeping any media involvement marking this unwanted milestone to a minimum".
Thursday 3 May 2007: Timeline
- 20:30 Kate and Gerry McCann leave their apartment to have dinner at a Tapas bar
- 21:05 Gerry McCann checks on Madeleine and her siblings
- 22:00 A man is seen carrying a child wearing pyjamas heading towards the ocean (E-fit images of the suspect were released as part of a 2013 Crimewatch appeal)
- 22:00 Kate McCann raises the alarm that Madeleine has gone missing
Officers have sifted through some 40,000 documents and looked at more than 600 individuals since the UK investigation was launched.
The Home office asked the Met to review the case and all the material gathered since 2007.
In March, the Home Office confirmed £85,000 was being given to the inquiry to fund the next six months.
Mr Rowley said four people considered as possible suspects in 2013 have been ruled out. Interviews took place at a police station in Faro, Portugal, and a large area of wasteland close to the Praia da Luz apartment was searched.
He said the enormity and complexity of the case had brought challenges. But the relationship with Portuguese investigators was "better than ever" and that was paying dividends in the progress made.
He said: "In a missing child inquiry every day is agony and an anniversary brings this into sharp focus.
"Our thoughts are with Madeleine's family at this time - as it is with any family in a missing person's inquiry - and that drives our commitment to do everything we can for her."