Jeremy Corbyn will lash out at the "hypocrisy and dishonesty" of Theresa May's Tories as he addresses one of Europe's biggest trade union events.
The Labour leader will use the Durham Miners' Gala speech to claim he is leading a "government-in-waiting" which could end austerity and scrap the cap on public sector pay rises but only if the party can stay united.
Mr Corbyn will tell the event the Grenfell Tower tragedy underlines the "terrible consequences" of cutting public spending.
He will say: "It is the hypocrisy that turns the stomach, when in the terrorist attacks or the Grenfell Tower fire, these Conservative politicians give warm words of praise and then a week later vote to cut those same people's incomes, having cut thousands of their colleagues' jobs already.
"People are fed up with the hypocrisy and the dishonesty of the Conservative Party.
"There's always the money available when they want to start another war.
"There's always the money for tax giveaways to big business or the very richest.
"There's always the money for new grammar schools or the latest hare-brained scheme to benefit their friends."
He will add: "We saw at Grenfell Tower the terrible consequences austerity and of doing things on the cheap."
Vowing to tackle inequality, he will say: "We will turn this monstrosity around and build a fairer, more equal Britain."
Labour's better than expected general election performance has secured Mr Corbyn's position but there is concern among some Labour MPs about moves which could make it easier to oust them in favour of candidates from the party's left.
Since the election Mr Corbyn has also been hit with a rebellion over Brexit which saw 49 of his MPs defy him by calling for the UK to stay in the single market and customs union.
The Labour leader will warn that indiscipline could jeopardise the party's chance of success.
"We are no longer just in opposition but a government-in-waiting," he will say.
"We have a huge opportunity now to change our country for the better if we learn the lessons of this great movement, stay united, stay disciplined and work together."
Mr Corbyn has spoken at the gala several times before, both as a backbench MP and Labour leader.
The event, also known locally as the Big Meeting, brings large crowds to the centre of Durham to watch the banners and brass bands parading through the city.
An estimated 200,000 people could gather in the historic centre to take in the spectacle and enjoy the huge funfair.
And many thousands gather on the Old Racecourse to hear the political speeches in the afternoon.
There will be additional security for this year's event with more road closures than in previous years and more armed police on visible patrol.
Meanwhile, Unite general secretary Len McCluskey will use the platform to demand Mrs May gives the same £1 billion deal to the English regions as she handed Northern Ireland.
He is accusing the Conservatives of "bunging" £1 billion to the Democratic Unionist Party in Northern Ireland while insisting there was no money for schools, housing and "sprinklers in tower blocks".
He was expected to say he was pleased for Northern Ireland and its communities.
"But if the Government can find money to invest in Northern Ireland, it must be found for all our nations and all our regions," he will say.
"A billion pounds would go a long way to helping regenerate industries here in the North East.
"It would help fill the gaping hole left when the Tories let a world-class plant at Redcar close its doors, throwing 2,000 skilled men and women out of work.
"So I demand the same deal for the North East as for Northern Ireland."