The parents of Madeleine McCann fear the costly police hunt into their daughter's disappearance could be shelved within three weeks by the new Home Secretary.

As funding is set to run out at the end of this month it has been revealed that detectives have still failed to unearth any significant clues during their latest trips to Portugal.

Maddie was snatched as a toddler from the family's holiday apartment in Praia da Luz in Portugal in May 2007.

As yet no fresh request has yet been made for more money from the Home Office, as another "D Day" looms for embattled Kate and Gerry.

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A source close to the couple from Rothley, Leicestershire, said: "They have been here before. They simply have no idea if the search will abruptly come to an end or will carry on. It is a daunting prospect they face once more."

Police officers on the scaled down Operation Grange inquiry have secretly continued to visit the Algarve searching for possible clues to Maddie's whereabouts but are believed not to be any step closer to finding her abductor, the Daily Mail reports.

The source added: "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."

The probe into Maddie's abduction was granted £150,000 (NZ$299,000) more funding by the Home Office in March.

More than £11million (NZ$22 million) has been spent so far on the probe to find the missing girl, who vanished 11-and-a-half years ago, and the investigation has repeatedly come under fire.

Since being launched in May 2011, officers have sifted (and translated) 40,000 documents produced by Portuguese police who conducted the initial investigation, and by the eight teams of private detectives who have worked on the case.

Some 600 "persons of interest" have been examined and "sightings" of Madeleine — in Brazil, India, Morocco and Paraguay, on a German plane and in a New Zealand supermarket — assessed.

The Portuguese investigation of Madeleine's disappearance was criticised by the British authorities as being not fit for purpose.

Detectives believe she was stolen by child traffickers, sex fiends or during a burglary gone wrong.

In May, her parents posted a poignant tribute to mark their daughter's 15th birthday - telling her: "We love you and we're waiting for you and we're never going to give up."

Gerry and his wife Kate wrote "Happy 15th Birthday Madeleine!" on the official Find Madeleine Campaign Facebook page.

The message accompanied a cherished last photo of her as a 3-year-old, smiling under a wide-brimmed sun hat and is posted alongside a green and yellow ribbon symbolising hope, strength and solidarity.

How the disappearance of Maddie McCann has unfolded over 11 years

2007

May 3: Gerry and Kate McCann leave their three children, including Maddie, asleep in their hotel apartment in Praia da Luz, Portugal, as they eat with friends in a nearby restaurant. When they return, they find Maddie missing from her bed.

May 4: A friend of the McCanns reports of seeing a man carrying a child away in the night. Meanwhile, airports and borders are put on high alert as search gets underway.

May 14: Robert Mural, a property developer who lives a few yards from the hotel, is made a suspect by Portuguese police.

May 30: The McCanns meet the Pope in Rome in a bid to bring worldwide attention to the search.

August 11: Police in Spain acknowledge for the first time in the investigation that Maddie might be dead.

September 7: Spanish police make the McCanns official suspects in the disappearance. Two days later the family flies back to England.

2008

July 21: Spanish police remove the McCanns and Mr Mural as official suspects as the case is shelved.

2009

May 1: A computer-generated image of What Maddie could look like two years after she disappeared is released by the McCanns.

2011

May 12: A review into the disappearance is launched by Scotland Yard, following a plea from then-Home Secretary Theresa May.

2012

April 25: After a year of reviewing the case, Scotland Yard announce they belief that Maddie could be alive and call on police in Portugal to reopen the case, but it falls on deaf ears amid "a lack of new evidence".

2013

July 4: Scotland Yard opens new investigation and claim to have identified 38 "people of interest".

October 24: A review into the investigation is opened by Portuguese police and new lines of inquiry are discovered, forcing them to reopen the case.

2014

January 29: British officers arrive in Portugal as a detailed investigation takes place. During the year, several locations are searched, including an area of scrubland near the resort.

2015

October 28: British police announce that team investigating Maddie's disappearance is reduced from 29 officers to just four, as it is also revealed that the investigation has cost £10million.

2016

April 3: Operation Grange is handed an additional £95,000 by Theresa May to keep the investigation alive for another six months.

2017

March 11: Cash is once again pumped into keeping the investigation alive, with £85,000 granted to keep it running until September, when it is extended once again until April next year.

2018

March 27: The Home Office reveals it has allocated further funds to Operation Grange. The new fund is believed to be as large as £150,000.