Ed Miliband brushed off Labour's shock defeat in the Bradford West by-election, claiming the party is set for a return to power at the next general election.
The party suffered a humiliating blow when Respect's George Galloway overturned a 5,000 Labour majority to storm home with a lead of more than 10,000 votes.
But in an interview with The Observer, Mr Miliband said that it was David Cameron and the Tories who were on the back foot after a week in which they were pilloried for the Budget and for sparking panic buying of petrol.
"I think it is going to be remembered for the end of the Cameron project," he said.
"Thursday night was a very bad result but there is a big picture about where politics is and I think people will look back on the last few weeks and say that was when the Cameron project hit the buffers and this was when Labour had their chance."
He added: "Opposition is a long and difficult haul. It is going to be a one-term haul, I am confident about that."
Following reports that the Tories tried to provoke a confrontation with the tanker drivers to create a "Thatcher moment" - echoing Margaret Thatcher's clash with the miners - Mr Miliband said the Government needed to come clean about its handling of the dispute.
"Over the last few days its every move has been designed to whip up unnecessary tension at the expense of the public," he said.
"Ministers knew all along that a strike could not possibly be less than seven days away even were it to be called - that is the law. Yet they panicked the nation all the way to the petrol pumps because they imagined it would boost them in the polls."