After weeks of an often bad-tempered and divisive party leadership contest that pitted Truss against Rishi Sunak, a former finance minister, the announcement at 9.30pm on Monday AEST will trigger the beginning of a handover from Boris Johnson.
He was forced to announce his resignation in July after months of scandal and will travel to Scotland to meet Queen Elizabeth on Tuesday to officially tender his resignation. His successor will follow him and be asked to form a government.
Long the front runner in the race to replace Johnson, Truss, if appointed, will become the Conservatives' fourth prime minister since a 2015 election.
Over that period the country has been buffeted from crisis to crisis, and now faces what is forecast to be a long recession triggered by sky-rocketing inflation which hit 10.1 per cent in July.
Foreign minister under Boris Johnson, Truss, 47, has promised to act quickly to tackle Britain's cost of living crisis, saying that within a week she will come up with a plan to tackle rising energy bills and securing future fuel supplies.
Speaking in a TV interview on Sunday she declined to give details of the measures she says will reassure millions of people who fear they will be unable to pay their fuel bills as winter approaches.
She declined to comment on a report that her energy plan could exceed STG100 billion ($A169 billion), but the lawmaker tipped to be her finance minister, business minister Kwasi Kwarteng, wrote on Monday that the government could afford to borrow more to fund support for households and businesses.
Truss has signalled during her leadership campaign she would challenge convention by scrapping tax increases and cutting other levies that some economists say would fuel inflation.
That, plus a pledge to review the remit of the Bank of England while protecting its independence, has prompted some investors to dump the pound and government bonds.
The Institute for Fiscal Studies cast doubt last month on Britain's next prime minister having room to make large, permanent tax cuts.
'Truss faces a long, costly and difficult to-do list, which opposition lawmakers say is the result of 12 years of poor Conservative government. Several have called for an early election - something Truss has said she will not allow.
Truss has said she will appoint a strong cabinet, dispensing with what one source close to her called a "presidential-style" of governing, and she will have to work hard to win over some lawmakers in her party who had backed Sunak in the race.
First, she will turn to the urgent issue of surging energy prices. Average annual household utility bills are set to jump by 80 per cent in October to STG3549 ($A5993), before an expected rise to STG6000 in 2023, decimating personal finances.
Britain has lagged other major European countries in its offer of support for consumer energy bills, which opposition lawmakers blame on a "zombie" government unable to act while the Conservatives ran their leadership contest.
In May, the government set out a STG15 billion support package to help households with energy bills as part of its STG37 billion cost-of-living support scheme.
