The House of Commons voted on Tuesday against reducing the voting age for a referendum on EU membership, blocking a move that might have boosted the campaign to stay in the 28-member bloc.
Members of parliament voted 303 to 253 to reject a move by the upper chamber, the House of Lords, to lower the voting age to 16 from 18 for the referendum which Prime Minister David Cameron has promised by the end of 2017.
The Lords is now unlikely to be able to force through the change after the Commons speaker invoked "financial privilege", which means that by convention the upper house should not overturn the lower house's decision.
Some MPs in favour of lowering the age said 16 and 17-year-olds should be able to vote because they would have to live with the referendum's consequences which could curtail their freedom of movement and complicate working in the bloc.
"The chance to extend the franchise for the EU referendum to 16 and 17 year olds would have led to a more democratic and a more engaging campaign," said Stephen Gethins, the European affairs spokesman for the opposition Scottish National Party.
"Today has put a stop to that hope," he said, calling Scotland's move to allow such teenagers to vote in its referendum on independence last year "a fantastic success" for allowing more people a taste of democracy.
Many analysts also say younger voters are more pro-European and could help the 'in' campaign, which is struggling to sell its message that Britons should be happy with the status quo.
Cameron is against lowering the voting age, which critics say would delay the timing of a referendum that will determine Britain's future role in world trade and affairs and also shape the European Union which has struggled with a series of crises.
He supports staying in a reformed European Union, but must convince the large eurosceptic lobby in his ruling Conservative Party that he has done enough to change the relationship they say is stacked heavily against British interests.
On Monday, the European Council president, Donald Tusk, warned Cameron that his central demand to curb immigration may be asking the bloc to do too much but said leaders could seal a deal in February.
Britain is split over membership of the European Union, but the 'out' campaign seems to be gaining the upper hand with many people put off by how the bloc has dealt with the arrival of hundreds of thousands of refugees and migrants in recent months.
