The Liberal Democrats have called on the Government to rule out scrapping safeguards that could lead to two million people falling off the electoral roll.
The Coalition Government introduced Individual Voter Registration, to put to an end the outdated system where a 'head of household' was responsible for everyone who votes at an address.
The move was backed up with £10m secured by Nick Clegg to ensure people registered to vote under the new system.
Crucially, the Liberal Democrats also secured a transition period where people would be eligible to vote until 2016 even if they were registered under the old system - an important safeguard that allowed time for people to understand and register on the new system.
The Conservatives insisted on a power for Ministers to be able to cut short the transition period at the end of 2015. But the Electoral Commission today said such a move risks seeing 1.9 million people fall off the electoral roll.
The Commission recommends leaving the extended timetable, secured by Lib Dems, in place.
Liberal Democrat constitutional affairs spokesman, Paul Tyler said:
"It's now clear that premature completion of the transition would drop nearly two million people off the register at the end of this year.
"These facts speak for themselves and show Liberal Democrats were right to insist on a long transition period to the new system.
"Ministers must now agree to let the transition takes its course, or face serious challenges to the legitimacy of the 2016 elections."