That means employees can receive 80 percent of their salary, with employers being asked to make contributions from the end of June that will increase incrementally until the end of the scheme.
Sunak is also set to announce in his budget that more than 600,000 people who were newly self-employed in the year 2019-2020 will be eligible to receive government support.
The Chancellor is set to announce that the Coronavirus Job Retention Scheme, which has been used by 11.2 million people since the beginning of the pandemic almost a year ago, will continue through summer and beyond when path out of lockdown is completed.
He is also expected to announce that the Self-Employed Income Support Scheme (SEISS) will be improved to allow nearly 600,000 people who were ineligible beforehand to claim cash grants.
That new support applies primarily to those who were newly self-employed in the financial year 2019-2020. Anyone who filed a tax return that year may now be eligible.
A fourth SEISS grant is also set to be announced, meaning that from next month self-employed people can claim 80 percent of three months' average trading profits up to a maximum of £7,500 in total.
The furlough scheme currently sees the government paying up to 80 percent of employees' wages for furloughed hours up to a cap of £2,500 per month, whilst the employer picks up National Insurance contributions and pension contributions.
From July 1, that government payment will decrease to 70 percent, with employers contributing 10 percent to make up the same amount, as well as the aforementioned contributions.
Then, on August 1, that government contribution will decrease again to 60 percent, meaning that 20 percent of unworked hours must be covered by employees, as well as pension and NI contributions.
Flexible furlough will still be allowed, meaning that employees can return to work for any amount of time or shifts whilst still claiming for unworked hours.
Sunak said: "Our Covid support schemes have been a lifeline to millions, protecting jobs and incomes across the UK.
"There's now light at the end of the tunnel with a roadmap for reopening, so it's only right that we continue to help business and individuals through the challenging months ahead - and beyond."
Sunak will announce in his budget that the UK will use it's full 'fiscal firepower' to support the country.
The three-part plan expected to be announced focuses on 'support, honesty, and building the UK's future economy', according to a statement from HM Treasury.
He will say: "First, we will continue doing whatever it takes to support the British people and businesses through this moment of crisis.
"Second, once we are on the way to recovery, we will need to begin fixing the public finances - and I want to be honest today about our plans to do that.
"And, third, in today's Budget we begin the work of building our future economy."
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