Lord Janner is among 24 eminent Jews who have fiercely criticised the Lithuanian government for celebrating the 70th anniversary of a provisional administration which undertook the "humiliation, harm, plunder and murder" of Jews in 1941.
In a petition emailed to ministers last Thursday, the signatories hit out at the Lithuanian government's "double genocide" campaign, which "distorts, obfuscates and undermines" the Holocaust and "negates the lessons of history of both the Nazi and Communist eras".
The petition stated that June 23, 1941 marked "the beginning of the end" of more than six centuries of Jewish life in Lithuania as the Nazi-aligned Provisional Government launched the "brutal destruction by genocide of a people so culturally and religiously rich that they made the city Vilna renowned as the Jerusalem of Lithuania".
By war's end, more than 90 per cent of Lithuania's Jews - almost 200,000 people - had been killed.
Aside from Lord Janner, who is also vice president of the World Jewish Congress, other UK-based petitioners included broadcaster Michael Freedland, Clive Sheldon QC and Central Synagogue's Rabbi Barry Marcus, a trustee of Yad Vashem UK.
The petition acknowledged that the "Lithuanian nation suffered for many years" under Soviet rule.