The UK is among a number of countries with military special forces operating inside Ukraine, according to one of dozens of documents leaked online.
It confirms what has been the subject of quiet speculation for over a year.
The leaked files, some marked “top secret”, paint a detailed picture of the war in Ukraine, including sensitive details of Ukraine’s preparations for a spring counter-offensive.
The US government says it is investigating the source of the leak.
According to the document, dated 23 March, the UK has the largest contingent of special forces in Ukraine (50), followed by fellow Nato states Latvia (17), France (15), the US (14) and the Netherlands (1).
The document does not say where the forces are located or what they are doing.
The numbers of personnel may be small, and will doubtless fluctuate. But special forces are by their very nature highly effective. Their presence in Ukraine is likely to be seized upon by Moscow, which has in recent months argued that it is not just confronting Ukraine, but Nato as well.
In line with its standard policy on such matters, the UK’s Ministry of Defence has not commented, but in a tweet on Tuesday said the leak of alleged classified information had demonstrated what it called a “serious level of inaccuracy”.
“Readers should be cautious about taking at face value allegations that have the potential to spread misinformation,” it said.
It did not elaborate or suggest which specific documents it was referring to. However, Pentagon officials are quoted as saying the documents are real.
One document, which detailed the number of casualties suffered in Ukraine on both sides, did appear to have been doctored.
UK special forces are made up of several elite military units with distinct areas of expertise, and are regarded to be among the most capable in the world.
The British government has a policy of not commenting on its special forces, in contrast to other countries including the US.
The UK has been vociferous in its support of Ukraine, and is the second largest donor after the US of military aid to Kyiv.
US Defense Secretary Lloyd Austin said the Department of Justice had opened a criminal investigation and he was determined to find the source of the leak.
“We will continue to investigate and turn over every rock until we find the source of this and the extent of it,” he said.