Factbox-English local elections: the main battlegrounds to watch

LONDON (Reuters) - Voters in England will take part in local elections, mayoral contests and a by-election to parliament on Thursday in what pollsters say could be the start of a breakdown of Britain's century-old two-party political system. 

The focus will be on whether Nigel Farage's Reform UK, whose pledges to reduce immigration and abandon climate-change targets mirror U.S. President Donald Trump's, can beat the governing Labour Party and opposition Conservatives in the contests.

Farage, a friend of Trump and a key Brexit campaigner, said he hopes a predicted big win for Reform will show he is building a political party capable of winning the next general election, which must be held at the latest in 2029.

Below is a rundown of the elections and what they might tell us:

RUNCORN AND HELSBY BY-ELECTION         

The first so-called by-election to parliament since Prime Minister Keir Starmer's Labour Party won last year's general election will be held in Runcorn and Helsby after a Labour lawmaker resigned following his conviction for punching a constituent.

Runcorn and Helsby in the northwest of England was one of the Labour Party's safest seats at the last election with the party winning 53% of the vote. Reform came second with 18%.

Reform is hoping to capitalise on unhappiness among voters with the government over recent tax rises, sluggish economic growth and the failure to stop the arrival of asylum seekers travelling in small boats on England's southern coast. 

Farage has visited Runcorn three times in recent weeks, but Starmer has not been to the area to campaign. 

Bookmakers show Reform UK have about a 70% probability of winning the by-election. 

ENGLISH COUNCIL ELECTIONS

There are more than 1,600 council seats up for election in England, with about a quarter of English voters eligible to cast a ballot.

Councils are the local government authorities with responsibility for providing public services that affect residents' day-to-day lives, rather than national issues like immigration and defence.      

Robert Hayward, a polling specialist and a Conservative peer, predicts Reform could win between 400 and 450 council seats, the most seats of any party.

Hayward predicts that the Conservatives would win between 375 and 425 seats and the Labour Party around 280 seats, close to its performance in local polls in 2021.

MAYORS

Voters will elect six mayors in different regions and cities across England.

Bookmakers expect the former Conservative minister Andrea Jenkyns, who lost her seat at the national election last year and defected to Reform, to be named the next mayor of Greater Lincolnshire, with responsibility for an area covering about a million people.

The betting odds also suggest Luke Campbell, a former Olympic gold medallist boxer and the Reform candidate to be mayor of Hull and East Yorkshire, and Alexander Jones, the Reform candidate to be mayor of Doncaster, will win.      

(Reporting by Andrew MacAskill; Editing by Ros Russell)