Burnham's Makerfield win triggers Greater Manchester mayoral by-election on July 30

Andy Burnham's return to Parliament has automatically triggered an election to find a new Mayor of Greater Manchester, with the vote set for July 30.

Sitting MPs cannot serve as regional mayors because the role incorporates the position of police and crime commissioner, creating a conflict of interest with national lawmaking.

The cost of finding a new mayor is estimated at £4.7 million, which was one of the reasons Labour gave for blocking Burnham's bid to become its candidate in the Gorton and Denton by-election in February.

For residents in Makerfield, it will be their third trip to the polls in three months, having already voted in May's council elections and Thursday's by-election.

What did Burnham say about leaving?

In his victory speech in Wigan, Burnham said: "It's with some sadness this result brings an end to my wonderful nine years as mayor of Greater Manchester. This city region has given so much to me, and it's a wrench to have to leave the job you love. But I'm not leaving the service of Greater Manchester."

He added: "I've always been clear it can't achieve everything it should, and we can't close the north-south divide, and we can't make all the great northern cities be what they should be without big changes at the national level."

What does the mayor do?

The mayor chairs a cabinet of the leaders of all 10 Greater Manchester boroughs. The role includes managing police, crime and fire services, overseeing transport budgets and the integrated Bee Network, and sharing responsibility for the £6 billion health and social care budget.

Under these powers, Burnham introduced free travel for 16 to 19-year-olds, a half-price pass for 18 to 21-year-olds, and took control of the region's bus network. He told a BBC debate during the campaign that the Bee Network "could not legally be overturned by a future mayor."

What voting system will be used?

The election will use the supplementary vote system rather than first-past-the-post. Voters will choose a first and second preference. If no candidate wins more than 50% of first-preference votes, the top two go into a run-off using second preferences from eliminated candidates.

The Electoral Reform Society has said the system "encourages a more positive style of campaigning as candidates try to win the second preferences of voters outside their usual support base."

Political parties now have just over a month to choose their candidates and campaign.