A senior policeman investigating the disappearance of Ben Needham on a Greek island has been summoned back to the UK amid reports members of his team went on an eight-hour drinking session.
Police are investigating a report in The Sun that officers spent an evening drinking wine and beer after launching a new appeal for information.
Ben, from Sheffield, was 21 months old when he disappeared on 24 July 1991.
His mother Kerry Needham said she "fully" supported the police team.
Det Supt Matt Fenwick, of South Yorkshire Police, is returning to the UK to discuss the matter with the force.
In a letter to South Yorkshire Police, Ms Needham's solicitor said her client "fears the entire investigation is in jeopardy".
Ms Needham told BBC Look North: "[It's an] absolutely insensitive, malicious thing to do to the detectives.
"They're out there working so hard on the investigation, then something like this could blow it for us. I'm so angry, it was really unnecessary."
The Sun has not commented.
South Yorkshire Police said "an internal investigation" would be held.
Officers are on the island for two weeks to re-appeal to local people and follow up lines of inquiry.
At a news conference on Tuesday, they offered a reward of 12,500 euros (£9,800) to anyone who provides information leading to the discovery of Ben.
Ms Needham has always maintained her son, who would now be 26, is alive and was probably abducted.
According to the newspaper, after the news conference at about 17:00 local time, the officers gathered for drinks at their hotel before heading to a pizzeria and bars.
The last members of the team finished the session shortly before 01:30, it said.
On social media, Ben's sister Leighanna said the report was "a slap in face to our family".
"Those detectives on that island are the only 'official' help we've had from this government in 25 years.
"And well done The Sun. You've well and truly slapped us in the face."
Earlier this year, the force secured £450,000 from the Home Office to help in the search.
'Working really hard'
Ben vanished after travelling to the island with his mother and grandparents who were renovating a run-down farmhouse building in the village of Iraklis.
Det Supt Fenwick told the paper the team was "desperately" trying to find the youngster and that officers were "working really hard".
He said: "We were up at seven this morning and we've got officers still out now (at 20:30) interviewing people.
"What I can say is that no public money was spent on alcohol."
A South Yorkshire Police spokeswoman said: "We're absolutely committed to this investigation and require the highest possible standards from all of those involved. The team must now get on with the important task in hand.
"Following concerns raised regarding the behaviour of some of those involved, the senior officer has been recalled to the UK with immediate effect to provide the details of exactly what has happened. An internal investigation will follow."
Officers travelled to Kos on Sunday to carry out "house-to-house visits" and distribute leaflets and posters.
A DNA test was carried out on a man in Cyprus in an attempt to establish if he could be Ben, but it proved negative in 2013.
In 2012, a police operation focused on a mound of earth and rubble close to where the toddler was last seen, but no trace of him was found.