Officers were preparing to start searches at the Arade dam about 30 miles inland from the beach resort where Madeleine went missing from a rented apartment while on a family holiday, authorities confirmed.
Police erected two tents beside the dam on Monday, Reuters TV footage showed.
Portugal’s Policia Judiciaria force confirmed the move in a statement on Monday night.
“In view of the news made public, the Judiciary Police confirms that, regarding the investigation into the disappearance of an English child, which occurred in the Algarve in 2007, steps are still being taken to fully clarify the situation.
“Within the scope of international cooperation, in the next few days in the Algarve region, new searches will be carried out, coordinated by the Judiciary Police, at the request of the German Authorities (BKA) and with the presence of the British Authorities.
“Information will be provided in due course on the outcome of the proceedings.”
The force made its statement as German police arrived at the reservoir where Tuesday’s search will start.
They were travelling in three Volkswagen vans, two black and one dark-blue, and a Ford Ranger with a Portuguese police escort in front.
They arrived around 6.30pm local time. It was the first time all day any German officials had been seen at the reservoir, where Portuguese police and civil protection workers arrived around lunchtime to prepare the ground for the searches.
German citizen Christian Brueckner, 45, was made a formal suspect, or an “arguido”, by Portuguese prosecutors in 2022.
The search has been requested by German authorities as Brueckner reportedly visited the area around the time Madeleine disappeared.
Brueckner is currently in prison for the September 2005 rape of a 72-year-old woman in the resort where Madeleine vanished,
He has never been charged over Madeleine’s disappearance and has denied any involvement.
A source familiar with the investigation said the operation in Silves municipality was being carried out at the request of German authorities.
Scotland Yard detectives are understood to have travelled to the area but only with a “watching brief.” The Metropolitan Police said it was referring all inquiries to the German authorities when contacted by The Standard on Monday afternoon.
Madeleine was three years old when she vanished from her mother and father’s holiday apartment in Praia da Luz in May 2007.
The search is set to last for at least two days and beyond that if anything of relevance is found.
It will be the first major operation of its kind since June 2014 when British police were given permission to do digs in Praia da Luz that involved sniffer dogs trained in detecting bodies and ground-penetrating radar.
This time round expert divers are set to explore the murky depths of the reservoir but digs will also take place in woodland by the water.
The area which will be searched is the Barragem do Arade reservoir near Silves where a lorry driver says he saw a woman handing a child like Madeleine over to a man two days after she went missing.
It was searched twice in February and March 2008 by divers hired by a Portuguese lawyer.
Marcos Aragao Correia organised the privately-funded operation after claiming he had been tipped off by underworld contacts that Madeleine had been murdered and her body thrown into the reservoir within 48 hours of her disappearance.
Two bags containing small bones were found during the second search after divers had earlier recovered several lengths of cord, some plastic tape and a single white cotton sock.
Portuguese police were alerted following the discovery but subsequently ruled out the possibility the bones were human because of their size.
Madeleine’s parents Gerry and Kate McCann had previously dismissed Mr Correia as a self-publicist and said there was no evidence suggesting any link between their daughter and the reservoir.
It is not thought to have been searched since March 2008 as part of the ongoing investigation into her disappearance.
A peninsula just over a mile long jutting into the reservoir was sealed off just after midday on Monday.
Several local council lorries were spotted on the piece of land as tents and other structures were erected.
Behind them in another area just under a mile away Portuguese Civil Protection workers were seen setting up a large blue tent alongside police who had mounted their own blue tent.
An onlooker said: “There are around two dozen Policia Judiciaria officers who seem to be supervising things at this stage.”
It was not immediately clear what had prompted this week’s operation, and whether it was hastily-arranged after police in Germany received a new tip-off or had been organised over time as prosecutors there continue to try to build up a picture of sole suspect Brueckner’s life on the Algarve and the places he frequented.
One well-placed source described it as a German-led operation which was being coordinated with the help of Portuguese police and was linked to information the authorities had gleaned pointing to regular trips Brueckner had made to the dam.
The insider said: “Portuguese police will offer logistical support but the initiative is German and they were the ones that requested the work that is going to take place this week.
“Scotland Yard officers are due to travel to Portugal but theirs will be a watching brief.”
Earlier this month Madeleine’s parents posted a poignant tribute to mark their daughter’s 20th birthday, telling her: “We love you and we’re waiting for you. We’re never going to give up.”
They added: “Happy Birthday Madeleine. Still missing. Still very much missed. Still looking. For as long as it takes.”
The message on the official Find Maddie Campaign Facebook page accompanied a cherished last photo of her as a three-year-old, beaming in a pink sunhat and clutching tennis balls.
Mr and Mrs McCann are expected to be kept informed of any developments as a result of the new search through Scotland Yard liaison officers.
German prosecutor Hans Christian Wolters has consistently said he is convinced Madeleine is dead.
Brueckner has yet to face any formal accusation over the youngster’s disappearance.
Last April he was made an official suspect or arguido in Portugal over Madeleine’s disappearance, although his defence lawyer Friedrich Fulscher labelled it a “procedural trick” linked to statute of limitations legislation at the time.
Last autumn Bruecker was charged in Germany with several sex crimes on the Algarve against women and children including the rape of an Irish holiday rep in 2004 and the sexual abuse of a 10-year-old girl on a beach near Praia da Luz in 2007.
Brueckner’s lawyer revealed in April those charges had been dropped against him in a bombshell development after successfully arguing prosecutors had no jurisdiction over him in Braunschweig where the Madeleine case was being brought.
The case could end up with prosecutors in Saxony-Anhalt instead but an appeal lodged by the Braunschweig public prosecutor’s office has yet to be decided.
Braunschweig prosecutor Mr Wolters has insisted they are still in control of the Madeleine McCann investigation and Bruecker, who denies any involvement in Madeleine’s disappearance, remains in custody.
In July 2020 Portuguese police and firefighters searched three wells for Madeleine’s body but failed to find any trace of her.
The abandoned wells are a 15 minute drive from a cottage Brueckner rented on the outskirts of Praia de Luz, on a narrow road leading down to a beach where the paedophile used to park his VW camper van.
The Scotland Yard digs nearly nine years ago in Praia da Luz were linked to the leading UK police theory at the time Madeleine died during a break-in and burglars dumped her body nearby.
The searches failed to find any trace of the missing youngster.