Opposition parties in South Korea say they will seek the impeachment of the acting president over his decisions on the country's corruption scandal.
Hwang Kyo-ahn decided not to extend a special prosecutor's investigation which involves his predecessor, President Park Geun-hye.
Prosecutors want more time to question her over the scandal.
Ms Park is awaiting a separate ruling on her December impeachment from the country's constitutional court.
She is still technically president, though she has been stripped of her powers while the constitutional court decides her fate. As long as she remains president, she is immune from prosecution.
But the wider corruption investigation which emerged from the scandal will now end on Tuesday, before special prosecutors have the opportunity to question Ms Park.
Any further investigation will fall to individual state prosecutors.
Mr Hwang, who remains prime minister while he sits in for the president, said that continuing the investigation is not in the best interests of the nation.
"After much deliberation (the acting president) has decided that it would be best for country's stability to not extend the special investigation and for the prosecutors to take over," his spokesman said.
Choo Mi-ae, leader of the main opposition Democratic Party, said Mr Hwang's decision indicated he was trying "to become Park's shield to protect her and her associates".
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Ms Park was impeached weeks after an old friend of hers, Choi Tae-min, was arrested.
In a written statement on Monday, Ms Park maintained her innocence, but said she had "belated regret, that I should have been more cautious with my trust in her."
Ms Choi is accused of using her presidential connections to pressure companies for millions of dollars in donations to two non-profit foundations she controlled. Ms Park is alleged to have been personally involved.
On 20 November, Ms Choi was charged with various offences, including abuse of authority, coercion, attempted coercion and attempted fraud, leading to the wider investigation.
A senior executive at electronics company Samsung was also arrested in connection with the probe.
Samsung is accused of giving donations to non-profit foundations operated by Ms Choi in exchange for government favours.
Ms Park's case at the constitutional court also heard the final arguments on Monday. It is not known when the final verdict will be delivered.
The court may reject Ms Park's impeachment, restoring her powers and returning the country to its status quo.
If, however they uphold the parliament's decision, a presidential election must be held.
Hundreds of thousands of protesters have taken to the streets calling on the ousted president to step down immediately, rather than continue to fight her impeachment in the constitutional court.