Last week, as the diplomatic stand-off between Britain and Russia continued over Moscow’s refusal to extradite the man suspected of the murder with radioactive material of former KGB agent Alexander Litvinenko, Gleb Pavlovsky, one of President Putin’s closest advisers, said Samuel Miliband fought under the command of Trotsky in the Twenties, eliminating White Russians opposed to communism.

Miliband’s grandfather died 40 years ago.

Pavlovsky is an influential figure in Moscow and director of a powerful political think-tank.

An investigation by The Mail on Sunday newspaper has established that Samuel was never involved in the killing of Russians.

“We have been told that Mr Pavlovsky’s comments - in a Russian newspaper - were simply an attempt to undermine Britain’s Foreign Secretary by highlighting his Jewish roots in a country where anti-Semitism is rife,” the newspaper said.

Samuel Miliband was born in the Jewish quarter of Warsaw in 1895 and had 12 brothers and sisters.

Eleven of them left Poland after the First World War, but one brother is thought to have joined the Red Army, fighting the Western powers in the Russian civil war.

After training as a leather worker in Poland, Samuel emigrated to Belgium in 1920.

He married Renia in 1923 and they had a son, Ralph, David Miliband’s father.

After WWII the family settled in London - but more than 40 members of the wider family were sent to their deaths and at least one relative is known to have perished at Auschwitz.

Samuel died from cancer in 1966, by which time Ralph had established an international reputation as a Marxist academic and Left-wing political theorist.

The 41-year-old David Miliband was appointed in June as Foreign Secretary by the new British Prime Minister Gordon Brown.