Ken Livingstone is expected to learn whether he is to be expelled from the Labour Party over his controversial claim that Adolf Hitler supported the creation of a Jewish state.
The former London mayor - who has been suspended since April last year - is facing a charge that he engaged in conduct that was "grossly detrimental" to the party.
It followed a radio interview in which he claimed the Nazi leader had supported Zionism in the 1930s before he ''went mad and ended up killing six million Jews''.
Supporters of the veteran left-winger say they expect the party's national constitutional committee - which has been sitting behind closed doors - to deliver its verdict at the end of the two-day hearing.
Arriving for the opening day on Thursday, Mr Livingstone insisted he had nothing to apologise for and that there had been "real collaboration" between the Nazis and the Zionist movement.
"I simply said, back in 1933 Hitler's government signed a deal with the Zionist movement, which would mean that Germany's Jewish community were moved to what is now Israel," he said.
"You had, right up until the start of the Second World War, real collaboration."