Mr Wickremesinghe was sworn in by President Gotabaya Rajapaksa as Minister of Finance and Economic Stabilisation and National Policies, two weeks after he was invited to form a unity government, following his predecessor's resignation.
On Tuesday, Mr Wickremesinghe told Reuters he would present an interim budget within six weeks, slashing infrastructure projects to redirect funds into a two-year relief programme.
The move to appoint Mr Wickremesinghe as finance minister comes as opposition parties refuse to take key Cabinet roles in the so-called unity government, highlighting the risk of further political instability.
“The president's party had wanted the finance portfolio, but the PM insisted he wanted it if he is to lead the country out of the economic chaos,” a senior politician involved in the negotiations told AFP.
As a member of the opposition, Mr Wickremesinghe criticised the president’s policies, highlighting falling foreign currency reserves and calling for the government to seek help from the IMF.
“Ranil Wickremesinghe is the better option, but we'll have to see if he has a majority in parliament and if he can do the job of both the prime minister and finance minister,” Lakshini Fernando, a macroeconomist for Sri Lanka-based investment firm Asia Securities, told Reuters.
The World Bank said late on Tuesday that it did not plan to offer new financing to Sri Lanka “until an adequate macroeconomic policy framework is in place”.
Mr Wickremesinghe has held talks with foreign envoys, including those from India, China and the US, and discussed with representatives from the Asian Development Bank and World Bank ways to replenish food, fertiliser and medicine supplies.
Sri Lanka needs $4 billion over the next eight months to pay for imports as it faces a severe shortage of foreign currency required to buy essentials such as fuel and medicine.
Meanwhile, public protests continue, calling for Mr Rajapaksa to step down following the resignation of his brother, and Mr Wickremesinghe’s predecessor, Mahinda Rajapaksa.
The island nation of 22 million people has fallen into default for the first time in its history after the expiry of a 30-day grace period for missed interest payments on two of its sovereign bonds.
The country’s central bank also gave a warning that headline inflation will worsen to 40 per cent over the next few months.
The World Bank is repurposing resources from previously approved projects to help the government with some essential medicine, temporary cash transfers for poor and vulnerable households, and to support farmers and small businesses.