Gordon Brown has thrust the issue of Tory party donations to the centre of the election campaign by declaring that the secrecy surrounding its biggest financial backer - Lord Ashcroft - is "a scandal".

In an exclusive interview with the Observer, in which he spoke at length of the need to restore faith in politics following the controversy over MPs' expenses, the prime minister attacked the lack of transparency over the peer's financial links to the Tories, saying it was profoundly wrong.

Delivering his strongest comments yet on the "Ashcroft question", Brown said it was now the duty of journalists and opposition politicians to "press these people for answers". "It's a scandal that we haven't had proper answers about where the [Ashcroft] money has come from and what the status of this person is."

The comments came as Brown, buoyed by last week's deal on the devolution of policing and justice in Northern Ireland and a narrowing of the Tory lead in recent opinion polls, predicted Labour could still win the next election - even with an overall majority. "I'm not complacent, but Labour can still win it," he said. "I'm absolutely sure of that."

He said that, as the economy emerged from recession, people would consider which party had made the right choices during the economic crisis. "I think people will look very carefully at us again and I think they'll make a decision. I've got faith in the good sense of the British people," he said.