Labour has taken two major scalps by winning the flagship Tory councils of Westminster and Wandsworth.
The totemic victories came amid fresh calls from Conservatives for Boris Johnson to quit as party leader.
In another boost for Keir Starmer, Labour gained Barrnet Council in north London from the Conservatives.
Labour has also won the new Cumberland Council, where all three local MPs are Tory.
Wandsworth, renowned for its low taxes, turned blue in 1978, a year before Margaret Thatcher, who considered the council her favourite, was elected prime minister.
“Boris Johnson losing Wandsworth is monumental. This was the Tories’ jewel in the crown,” a Labour source said.
London Mayor Sadiq Khan said that “history has been made” with the victory.
Khan tweeted a photograph of himself standing beside Labour councillor Simon Hogg, the council’s new leader.
Alongside the photograph, Khan wrote: “The three words we’ve wanted to hear for so long: WANDSWORTH LABOUR GAIN.”
Westminster has been held by the Tories since its creation in 1964, and was where Johnson cast his ballot yesterday.
In Barnet, the outgoing Conservative leader Daniel Thomas said: “I think this is a warning shot from Conservative supporters and I think our loss today is not only due to the fact that I have just mentioned but also a fair number of Conservative voters who just didn’t go out to vote, stayed at home.”
He added: “Clearly if Labour are to get a majority in Parliament they need to win Barnet. They won the council, if they win our parliamentary constituencies as well, then it doesn’t bode well for us to form a Government in future general elections.”
The Tories also lost Worcester to no overall control, with gains for the Greens and Labour.
Labour also gained Southampton from the Tories, who only won control of the council last year.
But there were signs Labour was struggling to make significant gains elsewhere outside the M25.
Meanwhile, the Lib Dems took Kingston upon Hull from Labour and were also edging towards winning West Oxfordshire from the Tories.
Lib Dem deputy leader Daisy Cooper said: “This is a great night for the Liberal Democrats. We’re making progress all over the country – building on our historic by-election victories last year.”
The elections are set against the backdrop of the partygate scandal and a cost-of-living crisis that could turn into a recession.
Council seats are also up for grabs in Scotland, Wales and many parts of England, while Northern Ireland is electing its new assembly in what could be the most significant vote of them all, with Sinn Fein seeking to become the largest party.
Many of the English seats were last contested in 2018, during Theresa May’s chaotic administration, meaning there may only be limited opportunities for opposition parties to make further gains.