Mr Corbyn issued his warning during a speech in front of a 1,000-strong crowd at the University of Leeds's Great Hall.
His 'missing millions' campaign is urging people to sign up to the electoral register before the changeover to individual registration takes place at the start of December.
At this point, residents must register by themselves rather than being registered by someone else in their household, or face being removed from the register.
Critics say "huge numbers" could be disenfranchised but ministers say "no genuine voters" will lose out.
To ease the transition to the new system, which was first agreed in 2013, nobody has so far been taken off existing electoral registers, but anyone who has not individually registered by 1 December will be removed.
Yesterday, Mr Corbyn warned that 80,000 people in Leeds risked falling off the Electoral Register when the changes come into force.
Labour says most of the people affected will be younger, or living in more deprived or urban areas, meaning these groups will be under-represented when constituency boundaries are re-drawn.
Mr Corbyn said yesterday: "They lose out and we lose out on their participation in the democratic process."
He added: "This is not boring, arcane stuff, it is the very lifeblood of democracy."
Earlier in the day, the Labour leader visited workers at Tata Steel in Scunthorpe.