Labour is on track for its worst local election performance, data analysed by the Guardian shows, in a blow that will pile further pressure on Keir Starmer’s leadership.

Barring a drastic change in fortunes, Labour’s vote-share could fall to historic lows across elections for councils in England and devolved parliaments in Wales and Scotland on 7 May, with big gains for Reform, the Greens and nationalist parties, according to recent polling.

The collapse in support is particularly existential in the race for the Welsh parliament, the Senedd, which Labour has dominated since its creation in 1999.

Polling shows Labour’s vote share falling by more than half in Wales, enough to push the party into third place, with Reform and Plaid Cymru vying for first.

Labour’s long-term decline in Scotland is expected to continue, with the Scottish National party likely to remain in power in Holyrood and Reform headed for second place.

In England, Labour faces several threats – from Reform, the Greens, the Liberal Democrats and independents – across 136 council races, including in its strongholds in London and the north.

While reliable polling across the council races is hard to come by, the recent fall in Labour’s national poll rating, alongside rises for other parties, is leading experts to expect “unprecedented losses”.

Stephen Fisher, a professor of political sociology at the University of Oxford, has estimated Labour will lose 1,900 councillors on 7 May – 74% of the number of seats the party currently holds that are up for re-election.

Such a result would be the worst local election performance for any prime minister since comparable data began.

Fisher estimates Reform will gain 2,260 councillors – which would triple the party’s local representation in England overnight – while the Greens will gain 450 and the Liberal Democrats 200.

The Conservatives are also set for a drubbing, with a net loss of 1,010 councillors under Fisher’s estimates, in a clear sign of voter dissatisfaction with Britain’s two main parties.

“Reform gains at last year’s local elections amounted to a record-breaking 41% of the seats up for election,” Fisher said.

“Now they have extended their polling lead, Reform should do even better this year if they can maintain the same conversion rate for opinion poll ratings into council seat gains.

“If they can, then the consequences are enormous losses for the Conservatives and unprecedented losses for Labour.”

A catastrophe of this scale for Labour could reignite the prospect of a leadership challenge against Starmer, who has continued to come under pressure over the appointment of Peter Mandelson as US ambassador.

In recent weeks several Labour ministers have played down the possibility of Starmer being removed because of any poor performance in the local elections, citing the international crisis over the Iran war.

But the Guardian’s revelation last week that Mandelson failed secured vetting for the US role has put Starmer’s future back in focus, amid calls for his resignation by opposition parties. Record-breaking electoral losses will only add to the concerns within Labour.

Yet, while there are clear signs 7 May could be an electoral catastrophe for Labour, it is less clear what this will ultimately mean for Starmer and his ability to remain in Downing Street.

On the one hand, the prime minister’s odds of survival have improved markedly since the US-Israeli war in Iran.

Starmer’s refusal to involve the UK directly in the war may explain why the level of public disapproval with his government fell throughout March.

On the other hand, that movement has already begun to reverse, and the polling does not yet reflect the full impact of the latest revelations over Mandelson.

Still, it raises the possibility that the prime minister has already reached peak unpopularity.

Labour figures will have to ask themselves: will the local elections be the worst it gets under Starmer, or a sign of more disasters to come?