The Government has been defeated - once again - this time over giving 16 and 17-year-olds a vote in the European Union referendum and letting 1.5 million teenagers have their say.
Peers voted 293 to 211 in favour of extending the franchise for the vote on whether Britain should stay in the union by the end of 2017. The 82 vote victory for the Opposition was heavier than forecast.
But the Government indicated it will not allow the move to stand, saying a major constitutional reform by reducing the voting age from 18 would not take place "through the backdoor".
The Government could ditch the amendment to the European Union Referendum Bill when it returns to MPs in the House of Commons, and send it back to the Lords - so-called "ping-pong".
The scale of the defeat, which follows the Lords embarrassing the Government over tax credits, may have made David Cameron think again.
A defeat of more than 40 votes - and in reality it was double that - was expected to prompt the Government to consider a U-turn.
But John Penrose, Minister for Constitutional Reform, said: "The age of 18, not 16, is widely recognised as the age at which one becomes an adult. Full citizenship rights – from drinking, to betting, to voting – should be gained at adulthood.
"It is inappropriate to try to change the nation’s electoral franchise through the backdoor of this Bill. To be seen to be free and fair, national referendums should be based on the prevailing general electoral franchise.
“The House of Commons has voted on three occasions in recent months against dropping the voting age from 18 - including overturning a Lords amendment just yesterday.
"The Government will re-affirm this clear position when the Bill returns to the elected Chamber and will seek to overturn this latest amendment from the Lords."
Labour's Shadow Foreign Office Minister Eluned Morgan said: “This is an historic victory. The House of Lords has given a vote of confidence in the ability of young people to make informed decisions on their future and this country’s future relationship with the European Union.
“The Prime Minister needs to listen and appreciate that the outcome of the EU referendum will affect these young people for far longer than him or anyone else in government. It’s an exceptional situation and a once in a life time vote – Mr Cameron should allow these young people their say and accept the Lords amendment.”
The preferred date for the vote among ministers is thought to be September 2016, but there have been warnings if 16 and 17 year-olds are given the vote the poll should be delayed by as much as 12 months to register them.
Some 'In' campaigners think the longer the debate goes on the more likely eurosceptics will triumph and Britain will vote to leave.
As such, Tory eurosceptics may be tempted to back the Lords when the Bill returns to the House of Commons, making it harder to overturn tonight's decision.
Tim Farron, leader of the Liberal Democrats, which in unison with Labour is forming a formidable Lords opposition to the Government, said: “The Liberal Democrats have been fighting for this for decades, and we are winning the argument.
“This is a victory for democracy, we will give over a million people a voice on their future.
"In Scotland 16 and 17 year olds proved that they have they not only have the knowledge but also the enthusiasm to have a say on their own future. Taking that away now would do them an injustice.
“The Government must now listen and act, Cameron cannot turn his back on 1.5 million young adults.”