“What dead Jew doesn’t have someone say Kaddish? What dead Jew doesn’t have a headstone that says something about him or her? I find the whole thing incomprehensible.”

So says Michael Lazarus, an 82-year-old retired businessman from Hertfordshire, who has been travelling to Belarus with his wife, Diana, every year for the past 14 years, to erect headstones to mark the mass graves of those who perished in the Holocaust.

In 2003, the couple founded the Simon Mark Lazarus Foundation, which was then joined by Americans Miles and Marilyn Kettler and Warren and Beverly Geisler to form the Committee for the Preservation of Holocaust Victims’ Memory.

The committee identifies the locations of mass graves and co-ordinates the construction of the memorials, each standing at almost six feet tall and inscribed in English, Belarusian and Hebrew.

Estimates of the scale of the murder of Jews in Belarus vary — in part because the country’s borders have changed dramatically following the Soviet invasion of Poland in 1939 — but Mr Lazarus has said there may be up to 2.5million eastern European Jews buried in thousands of sites, without any indication that the remains are there.

“The vast majority of something like 3,500 graves — Jewish graves — will never be marked, will never be known,” he said.

“It’s just unconscionable, it’s unsupportable. We’re Jews. And I think we actually owe it to those people to mark the spot and say: ‘This is a Jewish grave. And at this site somebody came and said Kaddish’.”

The couple, who are members of the Hadley Wood Jewish community, are now looking for others to take up the reins and mark the remaining sites.

Mr Lazarus called on young members of the Jewish community in the UK and elsewhere to become involved in the project, as well as the “large communal organisations and synagogues” to co-ordinate and fund it.

He said: “I think the big question is how the communal bodies, the synagogue bodies, can help.

“What we do need is the Jewish community here to see this and to want to do something about it — because they could do it.

“I think it’s our responsibility to do it and I’d like to think the Jewish community here and elsewhere will think it’s their responsibility, too.

“As long as we’re able to breathe we’re going to do this,” said a determined Mr Lazarus, “but we would like others to help.”

He said time is running out in the race to mark as many of the thousands of sites still without some form of commemoration, before the Holocaust becomes “just another piece of history”.

As it leaves living memory, Mr Lazarus said there was a “real danger” that the vast majority of these mass graves would never be marked.

The International Holocaust Remembrance Alliance (IHRA) said of these “killing sites” in Eastern Europe that they are “under-researched”, adding that it works to bring together individuals and groups — such as the Simon Mark Lazarus Foundation — to carry out memorial projects.