In a further set-back, officers have been told a request to search the homes of suspects who worked at the Ocean Club resort in Praia da Luz has been rejected.
The stand-off has come after the Met set up Operation Grange in 2011 to investigate the toddler's disappearance.
Portuguese authorities refused to mount a joint investigation, preventing Scotland Yard from carrying out its own activities in Portugal, and a number of requests have since been snubbed.
In a letter to the media yesterday, Met Police assistant commissioner Mark Rowley suggested the relationship between the forces was on a knife edge.
He warned officers could not keep the public up to date with information in case it upsets Portuguese authorities. 'The advice I am receiving from Portugal is that their approach is very different,' he said.
'If we provide any briefings or information on the work they are undertaking on our behalf activity will cease until that problem dissipates.
'We will not be able to provide any information concerning the activity because ultimately it could mean the work stops.'
Deputy assistant commissioner Martin Hewitt added: 'We have been issuing a number of letters of request. Some of the requests are not being acceded to.
'It's not fatal to what we are going to do - it's their judicial authority.' He said they were not being refused because of any lack of resources.
'That's causing us frustration because we know what we want to do and we are ready to go with that. But the process is the process,' he added.
Maddie disappeared on May 3, 2007, when she was three years old. Her 11th birthday is next week.
The cold case review into her disappearance has involved the painstakingly analysis of every document collected during the multi-million pound inquiry.
Appeals on BBC's Crimewatch led to new information and officers now plan to dig up key areas of Praia da Luz, just yards from where she vanished.
The key area to be searched is an overgrown wasteland, which was bought by Irish developers before being abandoned because of a property crisis in the area.
Tension between UK, Portuguese authorities
Metropolitan Police's Assistant Commissioner Mark Rowler has blamed the Portuguese authorities for the lack of information being made public by Operation Grange.
In an open letter dated today, the senior police officer said that despite pleas for the media to be briefed on ongoing developments in the search for Madeline McCann, Portugal's Policia Judiciara have threatened to stop work if information is passed on.
Revealing the ongoing tension between British and Portuguese police, Assistant Commissioner Rowley said he had explained to his opposite number in the Policia Judiciara that it is the Met's preferred practice to make public as much information and context as possible, providing it does not compromise the investigation.
He revealed that the reply from the Portuguese police was that they do not brief the media on an ongoing investigation, accompanied by a threat that if any information were to emerge, the work undertaken by Portugal on behalf of Scotland Yard would 'cease until that problem dissipates'.
'It is important you understand this and appreciate the position in which I find myself. We will not be able to provide any information concerning the activity because ultimately it could mean the work stops,' Assistant Commissioner Rowley said.
'We respect the Portuguese position as we would expect them to respect our position if we were carrying out work on their behalf in the UK,' he added.
Portuguese detectives will use radar equipment and mechanical diggers to excavate the site, while officers on foot will conduct fingertip searches.
Local reports suggested cadaver dogs could also be used in the searches - a controversial development considering the flawed initial Portuguese police case against the McCanns relied on evidence from two British sniffer dogs.
Also targeted will be an area by the beach, two locations in the surrounding countryside and land near Our Lady of the Light Church in Praia da Luz, where Gerry and Kate were given keys to pray alone after Maddie disappeared.
Met officers - who will pay for the work - will be in the area but can, at best, only supervise.
There are even concerns they may be banned from 'witnessing' the digs, with decisions on the details involved due to be made imminently by Portuguese authorities.
Respected Portuguese newspaper Jornal de Noticias wrote: 'The logistics necessary for the execution of all this work will be costly.
'It will be paid for by the UK which has already spent nearly seven million euros on this investigation.'
After 26 trips to Portugal to negotiate the work, British detectives have also been told they will not be allowed to raid the homes of three key suspects who worked at the Ocean Club when Maddie disappeared.
And, as none of the suspects is British, the Met does not have jurisdiction to make arrests.
The three former staff members, who are thought to have carried out break-ins at the resort, are considered 'people of interest' in the inquiry.
Analysis of mobile phone data suggests the men were close to the scene of her abduction and were in contact in the hours that followed.
One theory is that the burglars raided the McCanns' flat and were panicked into snatching the youngster when she woke up.
Met detectives reportedly requested the men's bank details and asked local officers to quiz the trio and search their homes.
But the judge who dealt with the requests 'did not consider Scotland Yard's arguments valid,' local sources said.
A spokesman for the Attorney General's Office in Portugal said: 'The content of the requests formulated by the British authorities is confidential.'
British police are said 'discreetly' to be on the ground in Portugal already, with a full team is expected to arrive in the coming days.
The Met Police said it was not prepared to provide a 'running commentary' on the inquiry.
- UK Daily Mail
