Sir Keir Starmer has been accused of "trying to rig elections" by handing the vote to 16 and 17-year-olds.
Labour plans to give voting rights to the two age groups before the end of 2025, a move that could add 1.5 million potential voters to the electoral roll.
Government sources have confirmed that MPs will begin voting on the proposals in the upcoming parliamentary session, starting this spring.
The measure is part of a wider elections Bill being drawn up by Deputy Prime Minister Angela Rayner's department, which aims to reverse some Conservative-introduced voter ID measures.
The legislation would mark the first change to the UK-wide voting age since 1969, when Harold Wilson's Labour Government lowered it from 21 to 18.
The proposal aligns with Sir Keir Starmer's election manifesto pledge to "increase the engagement of young people in our vibrant democracy".
The measure is part of a wider elections Bill being drawn up by Deputy Prime Minister Angela Rayner's department
PA
With Labour's current supermajority in Parliament, the measure is expected to pass through the Commons with ease.
The move comes just days after the Government announced the postponement of several local elections scheduled for May.
Reform UK leader Nigel Farage has accused Labour of attempting to "rig future elections" with the proposal.
"Labour have cancelled the votes of millions of people on May 1 and now we learn they are attempting to rig future elections with votes for 16 and 17-year-olds," he told The Telegraph.
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Reform UK leader Nigel Farage has accused Labour of attempting to 'rig future elections'
PA
Conservative Party chairman Nigel Huddleston called it "a desperate attempt to swing the next election in their favour," adding that Labour "can't decide whether they think 16 or 18 year-olds are adults".
According to Office for National Statistics data, the expanded franchise could bring 1.5 million extra voters onto the electoral roll.
However, youth voter turnout has historically been low, with only 37 per cent of eligible 18-24-year-olds voting in the last general election.
If similar turnout patterns emerge among 16 and 17-year-olds, this would translate to approximately 520,000 new voters.
Recent YouGov polling indicates these younger voters would predominantly support Labour, with around one in ten backing Reform UK, which has seen increased youth support.
Nigel Huddleston
PA
The controversy follows Rayner's recent decision to cancel elections for 5.5 million voters in councils due for boundary reorganisation.
Reform UK, which has recently topped some polls, claims this cancellation particularly impacts their expected surge in local council representation.
The party is now demanding transparency regarding the Deputy Prime Minister's communications with affected councils in East Sussex, West Sussex, Essex, Hampshire, Norfolk, Suffolk, Surrey, Isle of Wight, and Thurrock.
"Reform UK MPs are probing what conversations the Deputy Prime Minister has had with local authority leaders," a Reform spokesman said.
A Government spokesman defended the proposal, stating: "This Government was elected on a manifesto that clearly said we would legislate to give 16 and 17-year-olds the right to vote in all UK elections."
The spokesman emphasised their commitment to strengthening democracy and building "an informed, engaged electorate for the future".
The Government pledged to ensure the change is implemented successfully and to bring forward the legislation "in good time ahead of the next general election".
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