The Liberal Democrats' victor in the Dunfermline and West Fife by-election, Willie Rennie, said today that the voters had "sent a powerful message to the Labour government that will rock the foundations of Downing Street".
"This is truly a historic victory." he said. "I think Gordon Brown may be a little bit worried himself.
"This by-election will send shockwaves through Westminster. Labour has taken the people up and down the country for granted for far too long. Too much spin and not enough delivery.
"It's time that Tony Blair and Gordon Brown both got that message. This is a sensational victory."
Mr Rennie said his party had shown that it was the clear challenger in the Labour heartland.
He said on BBC television: "This election is quite symbolic. People are fed-up with Labour. They are going to be fed-up with Labour in their heartlands because Labour think they will get elected without doing any work at all and this shows that no longer is the case.
"The Labour Party must be very worried about the Liberal Democrat advance, especially right next to Gordon Brown's own seat.
"I've got three or four years to prove I am worthy to be the MP in Dunfermline and West Fife and I will be working hard to win more of their trust to make sure I win it next time. This seat can be held for the Liberal Democrats."
Asked who he was backing in the leadership contest, Mr Rennie said: "I've had quite a busy time over the last few hours and weeks and have not finally made up my mind who I'm going to be backing in this election."
Acting leader and leadership contender Sir Menzies Campbell said: "It comes, of course, as a great antidote to the turbulence of the last few weeks and will make an enormous difference to the morale of the party. The Liberal Democrats are back in business."
The Scottish Secretary Alistair Darling admitted it had been a poor result for Labour but said the campaign was dominated by local issues, including plans to quadruple the tolls on the Forth road bridge.
He said on BBC Radio 4's Today programme: "It was a very bad result for us, let's make no bones about it. This was a safe seat, we should not have lost it.
"We were the Government party, people coalesced around the Liberals, they wanted to send a very clear message. We have got to learn from that and make sure that we don't make that mistake again.
"It was the local stuff that actually did for us. People saw that the Tories were absolutely nowhere, the Nationalists have just flat-lined, they piled in behind the Liberal Democrat and we paid the penalty."