Results are coming in after local elections in England and Northern Ireland, with disillusionment about Brexit expected to be a big factor.
Voting took place for 248 English councils, six mayors and all 11 councils in Northern Ireland.
Minister James Cleverly said it would be a “tough night” for the Tories, who expect to lose hundreds of seats.
Labour councillors have also warned of a “very difficult night” for their party, with both sides blaming Brexit.
Results for 108 English councils are expected before 06:00, with the other 140 results expected throughout Friday.
The Northern Irish results will take longer to come in. No local elections are taking place in Scotland and Wales.
Local elections give voters the chance to choose the decision-makers who affect their communities, but national politics always forms the backdrop – and in this case, Brexit looms large.
BBC political editor Laura Kuenssberg says early results suggest both of the two main parties are being punished for their handling of it, with smaller parties, such as the Liberal Democrats, Green Party and UKIP, benefitting from their losses.
She says the Conservatives have been predicted to lose up to 800 seats, while the Lib Dems could gain as many as 500.
Polling expert Prof Sir John Curtice agreed it may be the smaller parties celebrating on Friday.
“One of the major features of the 2017 general election was that, between them, the Conservatives and Labour dominated the election, winning over 80% of votes – the biggest combined share since 1970,” he said.
“On these early signs, we might be saying tonight those days are over. The message of the opinion polls that both the Conservatives and Labour have been losing ground in the Brexit impasse might be confirmed by end of night.”
He also said independent candidates were a further indication of the apparent “plague on all your houses” mood of the polls, with an increase of 43 seats with just 43 councils’ results declared – three times what they had in the same seats before the vote.
The Conservatives have so far lost control of six councils – including Basildon and St Albans – with Labour losing control of two, Hartlepool and Wirral.
The Tories have won one council – Walsall – which no party previously had control of, and held Swindon, despite anger in the area about the loss of a major employer.
James Cleverly said he hoped local councillors would be judged on their individual performance, but told BBC News: “It is unrealistic for me to pretend that with nine years in government and Brexit as a backdrop, [it will] be anything other than a tough night for [the Conservative Party].”
Sir Graham Brady, who chairs the Conservative backbench 1922 committee, said Brexit had been a “massive frustration” for voters and affected the number of Tory supporters coming out.
He told the BBC: “We need to be more obviously competent as a government and we need to make progress – crucially on the central issue of Brexit, which is both frustrating people that it hasn’t happened and also taking up far too much time.”
Labour’s shadow international development secretary Barry Gardiner said his party may struggle in the polls to please both Remain and Leave voters.
“There are two competing principles here and we are trying to hold them in tension,” he told the BBC. “We are trying to say there is a way to reconcile them.
“If a party is seen to be speaking with two voices, it’s very difficult to communicate the policy.”
But Labour MP Jess Phillips said her party’s policy on Brexit had “failed” and they needed to show more “bravery” over the issue.
Liberal Democrat MP Ed Davey said his party was having an “awesome night” thanks to voters “rejecting” both Labour and the Conservatives.
UKIP’s Mike Hookem said his party was taking votes from both the Conservatives and Labour, adding: “We’ve had two years of turmoil. We’re back on the rise again.”
If you forget about Brexit for a moment – perish the thought – and apply some of the more traditional expectations to a night like this, what you would expect to see at this stage in the election cycle is a government losing lots and lots of seats and the opposition really making strides.
So far tonight though, it seems none of those things are really happening and Brexit seems to have changed yet another set of conventions.
What we are seeing in various different shades – whether it is support moving to the Lib Dems or support moving to the Greens – is people are almost saying, “as long as it is not either of the two main parties, I am happy to go with that.”
What it does mean in terms of the potential lessons for an eventual general election though is the results so far do not suggest that either party can break away from each other.
This is the biggest set of local elections in England’s four-year electoral cycle, with more than 8,400 seats being contested.
A further 462 seats are up for grabs in Northern Ireland.
Voter turnout has yet to be confirmed, but it often varies depending on what other elections are taking place on the same day.
Local elections in 2018 saw a turnout of just 36%, but in 2015 – when they coincided with a general election – turnout reached around 64%.
By 06:00 BST results from just under half of the English councils (108) are expected to have come in.
The remaining 140 are scheduled to come in throughout Friday, mostly between midday and 19:00 BST. Cheshire East is expected to declare last at 21:00.
The Northern Irish results will take longer to come through because of a more complicated voting system.
Of the 248 elections in England, 168 have been district councils which are in charge of setting and collecting council tax, bin collections, local planning and council housing.
There were also elections taking place for 47 unitary authorities and 33 metropolitan boroughs which look after education, public transport, policing and fire services, as well as all the services of district councils.
In Northern Ireland, councils are responsible for services including local planning and licensing, waste collection and enforcing safety regulations to do with food, workplaces and the environment.