Labour is heading towards its worst local election performance on record, according to analysis of polling data by the Guardian, increasing pressure on Prime Minister Keir Starmer’s leadership.

The projections suggest Labour’s vote share could fall to historic lows across council elections in England and devolved polls in Wales and Scotland on 7 May, with rival parties including Reform UK, the Greens and nationalist parties expected to make significant gains.

The most severe setback is expected in Wales, where Labour has governed since 1999.

Polling indicates the party’s support could fall by more than half, potentially dropping it into third place behind Reform UK and Plaid Cymru.

In England, Labour faces challenges across 136 council contests, including in traditional strongholds in London and northern regions, as voters shift towards Reform UK, the Greens, the Liberal Democrats and independents.

Electoral modelling by Prof Stephen Fisher of the University of Oxford suggests Labour could lose around 1,900 councillors — about 74% of its seats up for election.

That would mark the party’s worst local election performance under any sitting prime minister since comparable records began.

The analysis also projects major gains for other parties, including 2,260 councillors for Reform UK, 450 for the Greens and 200 for the Liberal Democrats, while the Conservatives are expected to lose more than 1,000 seats.

Fisher said Reform’s rising support could translate into “unprecedented losses” for both Labour and the Conservatives if current trends continue.

The expected results could intensify scrutiny of Starmer’s leadership, although some Labour figures have played down speculation about any immediate challenge, pointing to the ongoing international crisis linked to tensions involving Iran.

However, recent political controversy, including questions over the appointment of Peter Mandelson as US ambassador, has added to internal pressure within the party.

While polling suggests Labour’s position has weakened in recent months, analysts say it remains unclear whether the local elections will mark a peak in voter dissatisfaction or the beginning of further decline.