A single punter has invested $1 million on Bill Shorten to win the May 18 election in the single largest bet ever recorded by betting agency Ladbrokes.

The bet, only the second ever made by the NSW man with Ladbrokes, was put on Wednesday afternoon not long after Mr Shorten held back tears while discussing his mother and a story about her by The Daily Telegraph.

A punter has backed Bill Shorten to be the next PM to the tune of $1 million - the largest bet ever recorded by betting agency Ladbrokes. Credit:Dominic Lorrimer

It eclipses the previous record bet at Ladbrokes of $370,000 earlier this year on Winx.

"We've obviously seen some sizeable bets come through in recent times, many of them on Winx, but clearly this gentleman thinks the Labor Party is a safer bet than the now retired mighty mare,"

Ladbrokes Australia chief executive, Jason Scott said.