The Government was challenged last night to lower the voting age across the UK after it dropped its opposition to 16-year-olds taking part in the planned referendum on Scottish independence.
Supporters of the move believe the concession to Scottish voters will revive the controversy over when teenagers should be allowed to participate in general elections. Senior Conservatives are hostile to the plan, although Tory sources acknowledged that the decision would put the issue back on the constitutional agenda.
Last night critics accused the Conservatives of acting in self-interest because young adults are less likely than any other age group to support the party. The Tories picked up just 30 per cent support among 18 to 24-year-olds at the last election, compared with 37 per cent across the electorate as a whole. It was the only group where the Conservatives lagged behind Labour.
The Liberal Democrats are in favour of lowering the franchise and Labour is expected to support the step its next manifesto. The move would boost the British electorate by about 1.5 million and proponents believe it would help to engage younger people in the political process.
Mr Cameron will meet the Scottish First Minister, Alex Salmond, next week to finalise the terms of the referendum which will take place in the autumn of 2014. Under an agreed broad outline, Scottish voters will be asked a simple yes/no question of whether they want to remain in the UK, which is a key demand of the Westminster Government.
In return, the electorate for the referendum will be extended to 16 and 17-year-olds, which the Scottish Nationalists believe will increase the chances of the independence vote being carried.
The Labour MP Natascha Engel said: "I'm really excited about what's happening in Scotland. People should have the right to have a say over the issues that affect them – it's a real denial of fundamental human rights for people who are 16."
Willie Sullivan, the Scottish director of the Electoral Reform Society, said: "If 16-year-olds are to be granted a vote on the future of the Union, it would be ridiculous not to give them a say on the trifling matter of their next local MP."
But a Conservative minister said: "We will oppose it for Westminster, we will say 'no' to that idea." A Downing Street source added that there were "no plans" to look afresh at the voting age.
Mr Cameron appealed to his party to campaign against Scottish independence with "everything we've got". He said: "What could matter more than saving our United Kingdom?"