Three former British special forces soldiers are feared to have been killed in a Russian airstrike in Ukraine, the Mirror reports.
It is understood the men died after the blasts near the Polish border, which has left possibly over 100 people dead.
Sources told Daily Mirror journalists the former troops were not part of the foreign fighters unit that was being trained at the now destroyed Yavoriv base.
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It is not known which branch of special forces they had served in.
As many as 30 cruise missiles were unleashed on the base, 35 miles from the western Ukrainian city of Lviv where thousands of refugees have flocked to.
It is believed all of the missiles were fired by six Russian bombers from within Russia, although a number of them were shot down by Ukrainian ground defence systems.
Asked in the aftermath of the devastating blasts on Sunday if British casualties were among the dead UK Foreign Office sources said they were unaware of any.
The British MoD confirmed there were no serving personnel killed or injured in the explosions.
A source told the Daily Mirror: "There were many more killed within the site than has been claimed and bodies are still being found.
"I do not believe the three British ex-military personnel would have known anything about it, fortunately.
"This is extremely sensitive as there are believed to be no serving British military personnel inside Ukraine as politically that would be extremely controversial."
Witnesses have described how the attack at 5.45am "turned the sky red."
Hours after the attack, ambulances were still rushing to the scene.
Roads leading to the facility were blocked with checkpoints and authorities were conducting search-and-rescue operations.
The base is known as the International Peacekeeping and Security Center.
The UK Foreign Office said it is urgently investigating.
It advises against all travel to Ukraine.
Ukrainian officials have said 35 people were killed in the strike at the base on Sunday.
Moscow said up to 180 "foreign mercenaries" died and a large number of foreign weapons were destroyed.
Ukraine also reported renewed air strikes on an airport in the west of the country.
The number of refugees fleeing Ukraine since Russia invaded on February 24 climbed to more than 2.8 million, United Nations data showed on Monday, in what has become Europe's fastest growing refugee crisis since World War Two.
European Union officials have said 5 million may end up fleeing while others have put the figure higher.
Millions of people have also been displaced inside Ukraine, with many evacuated only as far as the quieter western regions, including to cities like Lviv.
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