Polls have closed across England, Scotland and Wales for local, mayoral and parliamentary elections, with the first results to be announced within hours.

More than 30 million people across Britain were given the opportunity to vote on Thursday in what is widely seen as the biggest test for Keir Starmer since the 2024 general election. Results across three nations could fundamentally change the political landscape and could have repercussions for the prime minister.

In what is seen as the first true multiparty battle, Reform UK and the Green party are expected to make significant gains at the expense of Westminster’s two biggest parties in England, while the Liberal Democrats could turn swathes of local government yellow and increase their total number of councillors for an unprecedented eighth set of local elections in a row.

The elections cover 136 local councils in England, with 5,014 seats being contested, including every seat on all of London’s 32 borough councils, more than a dozen borough councils, six unitary councils, six county councils and three district councils. A further 73 councils are holding elections for half or one-third of the seats available.

The first results are expected at about 12.30am, with a further glut arriving from about 3am onwards. About a third of councils should have declared results by 7am on Friday, while the most significant results – including mayoral results in London boroughs and council results in the major cities of Manchester and Leeds – should start to come in at lunchtime.

By the end of Friday about 80 more councils will have declared results, but the final councils – including Croydon and Tower Hamlets in London, as well as Hastings in Sussex – will not declare until Saturday afternoon.

Results in Scotland and Wales should become clear by around 4pm on Friday, with more local election results announced in the late afternoon and early evening.

Counting for mayoral elections will only begin on Friday, with Hackney and Newham expected to declare at 1pm, followed by Watford at 2pm, Lewisham at 3pm, Croydon at 4pm, and Tower Hamlets at about 6pm.

Labour strategists will await results with some trepidation, with the party resigned to potentially record-breaking losses. By some estimations Labour could lose more than 1,800 seats – 75% of those it is defending. It has faced pressure from Reform across the former “red wall” in the north-east, Midlands and north-west of England, while in London – where four years ago Labour swept to its best performance since 1971 – the party has been challenged by the Greens and independent candidates.

Labour could lose affluent boroughs such as Wandsworth and Westminster back to the Conservatives, and Merton in the increasingly Liberal Democrat-dominated south-west. Reform has been campaigning heavily in London’s largely Conservative outer ring in places such as Bexley, Bromley, Barking, Hillingdon and Havering, as well as Labour-run Barking and Dagenham.

Zack Polanski’s Green party has expressed confidence in winning the Labour-run councils of Lambeth, Islington, Southwark and Hackney, and has campaigned hard in Lewisham and Camden, where Starmer is an MP.

The Green party also hopes to capitalise on its recent success in the Gorton and Denton byelection and has targeted Manchester city council, where a third of councillors are up for election. There has also been sustained campaigning in other cities with large student populations, such as Cambridge, Oxford, Leeds, Sheffield and Reading.

Reform is expected to make gains in English councils, including traditional Labour strongholds such as Sunderland and traditional Tory enclaves such as Essex, with anything other than control of a series of councils likely to be viewed as a poor performance.

Reform is toe to toe with Plaid Cymru in Welsh Senedd elections and could emerge as the SNP’s main opposition in Holyrood.

The battle on the right will be seen most clearly in the six county councils that last went to the polls during Boris Johnson’s 2021 “vaccine bounce”. Reform expects to take control of the council in Essex and aims to win traditionally Tory-dominated Norfolk and Suffolk. The Liberal Democrats hope to make inroads in Hampshire, West Sussex and two new councils formed in Surrey at the Conservatives’ expense.

While attention has focused on insurgent parties, the Lib Dems have an outside chance of becoming the biggest party in English local government.

In Scotland, Labour could be pushed into third place behind the SNP and Reform. Polling before the vote suggested the SNP was on track for a fifth term in office, but it is unclear if any party will win the 65 seats needed for a majority in the 129-member Scottish parliament.

In Wales – where a new proportional voting system will be in place – voters will elect 96 politicians across 16 constituencies, with six members of the Senedd (MSs) elected in each. Welsh Labour is facing the prospect of a historic defeat, after coming first in Wales in every general election since 1922 and every devolved election since 1999. Plaid Cymru and Reform UK are neck and neck in the latest poll, although coalition maths make it highly unlikely Reform would be able to form a government.