Former treasurer Peter Costello backed Donald Trump as the slight favourite to win the upcoming US election but hoped that regardless of the outcome, it would not be a tight race.

Costello warned that a tight race in a worst-case scenario could inflame the US government system with either major party potentially contesting the verdict due to growing divisions in the country.

“I think probably Trump wins by a whisker,” Costello told a Property Council conference.

“I’ve got to make another point here because I think it’s an important point. The trouble is going to be this. It’s going to be so close and the US is so deeply divided, is either party going to accept the other party winning?

“This, to me, would be the worst outcome if America turns on itself after this election. It will be nice if somebody won so clearly that you couldn’t argue afterwards, but I’ll be very worried if there’s instability after this election, which could go on for some time. You know what the US system is like, they’ll go through the courts. It’ll go on for years.”