Scotland Yard detectives have spent time in Spain as part of their re-examination of the disappearance of Madeleine McCann in Portugal.
The Metropolitan Police joined the hunt for the missing girl, who vanished in 2007, following a request by Home Secretary Theresa May made with the backing of Prime Minister David Cameron.
The force has now revealed that detectives working on the case flew to Spain as part of their review and spent three days with police in Barcelona.
Kate and Gerry McCann arriving at the Leveson inquiry last month
Officers working on the case also travelled to Portugal three times as they continue to hunt for leads.
The Met has vowed there will be no limits in its re-examination of how Madeleine disappeared from a holiday flat in Praia da Luz in the Algarve.
A huge hunt was launched after she went missing on May 3, 2007 while her parents Kate and Gerry dined with friends nearby but she has never been found.
Her frustrated parents, both doctors who live in Rothley, Leicestershire, have battled to retain the case's high-profile.
Read more on Martin Brunt's blog
"We are grateful to them and to the Metropolitan Police Service for undertaking this fundamental and highly valuable procedure which has been underway now for several months.
"Although it will be a lengthy and difficult process, it is definitely a major step forward for Madeleine."
Detectives refused to reveal what led them to Barcelona between November 23 and 25, but a Scotland Yard spokesman said: "The review continues.
"There has been very good co-operation with Portuguese authorities and liaison will continue. We are not prepared to discuss specific details of these visits nor speculate about any future deployments to Portugal or elsewhere."
Detective Chief Inspector Andy Redwood, the senior investigating officer, is understood to have been among a team of detectives who went on the trips.
Madeleine McCann was snatched from an apartment in Praia da Luz
Sky's crime correspondent Martin Brunt says police are not believed to have made a breakthrough and are still doing the groundwork of liaising with foreign officers.
A total of 30 people are now working on the force's Operation Grange.
The review prompted criticism when it was launched in May, with politicians expressing concerns that it would divert resources from other crime victims.
Portuguese detectives, helped by officers from Leicestershire Police, carried out a massive investigation into Madeleine's disappearance.
But the official inquiry was formally shelved in July 2008, after which no police force was actively looking for her.