It's a day of shock resignations in politics as two world leaders announce they are stepping down

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After a year of surprises in the UK and Europe, the world's political landscape continued to shift with the resignation of two world leaders.

If Brexit hadn't rocked the EU boat enough, it now faces fresh turmoil after Italy's prime minister Matteo Renzi announced his resignation following a decisive defeat in his constitutional reform referendum.

Ninety minutes after polls closed, the 41-year premier conceded defeat as exit polls suggested 57% of voters had rejected his plan to streamline the country's system of government.

The result will be seen as a further victory for the anti-establishment backlash sweeping much of Europe and America - with the opposition to his plan headed by the anti-euro Beppe Grillo's Five Star Movement.

Italian PM Matteo Renzi.
Matteo Renzi (Gregorio Borgia/AP)

The populist opposition says it is ready to run the country - elections are due in spring 2018, but with Renzi stepping down, they are now likely be held some time in 2017.

Meanwhile, on the other side of the world, New Zealand's popular prime minister John Key stunned the nation by announcing he will resign, after eight years as leader.

New Zealand PM John Key.
John Key (Manish Swarup/AP)

While Renzi's decision to step down was a result of his country's referendum, the New Zealand PM's reasons to quit appear to be more personal.

Key, 55, who had been widely expected to contest his fourth general election next year, said he wanted to ensure he did not make the mistake of other world leaders, and instead wanted to leave while he was on top of his game.

Key said he had made personal sacrifices for the job and the role had taken a toll on his family.

He said his National Party caucus would meet on December 12 to decide on a new party leader and prime minister - and he backs his deputy Bill English for the role, in shock news that sent the New Zealand dollar falling by nearly 1%.

New Zealand' deputy PM Bill English.
Bill English (Nick Perry/AP)

Key said he was not sure what life after politics would bring, other than he would probably take up positions on a couple of boards. He said he would remain on as an MP long enough that he wouldn't force a special election ahead of next year's general election.

"All I can say is that I gave it everything I had," he said. "I have left nothing in the tank."