The older Su said: “In order not to create any trouble for President Tsai and to allow the prosecuting authorities to investigate the case in a just and fair manner, I hereby announce my resignation as the presidential secretary general as of now.”

The nephew has been questioned by prosecutors over allegations that he had taken bribes totalling NT$20 million (US$680,000) from a businessman identified as Lee Heng-lung since 2013 to help him in a takeover battle.

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He is one of five current and former legislators who have been questioned in the investigation. Prosecutors have now asked the courts to detain them.

The others – Chen Chao-ming and Liao Kuo-tung from the main opposition party the Kuomintang, the chairman of the smaller New Power Party Hsu Yung-ming and former DPP legislator Mark Chen – are accused of taking bribes ranging from tens of thousands of Taiwan dollars to NT$2 million, according to prosecutors.

All five deny the allegations. Prosecutors are also seeking to detain Lee, who denies wrongdoing and says he only lent money to Su Chen-ching.

Insisting that he was clean, Su Jia-chyuan said he and his wife have never been investigated or charged with corruption during his political career.

“Integrity is my second life, which I have vowed to resolutely protect, but recently there have been smears or distortion of the facts to accuse me of trying to use my position to benefit my family,” Su said, adding he would not allow malicious acts to ruin his good name.

“I tried my best to play the role of the president’s chief of staff … but my family member’s [alleged] involvement in the case has brought trouble to President Tsai, for which I feel deeply sorry,” Su continued.

In a separate case, prosecutors have said they are investigating allegations raised by the KMT that Su’s nephew had asked him to help him in a business deal with the Indonesian authorities. Both Su and his nephew have dismissed the allegations as groundless.

On Monday, Tsai said she respected Su’s decision after he said he did not want the case to harm the administration.

“What I want to say is for those in the top position to have the strongest demands about themselves in terms of personal behaviour,” she said, adding what the official said and done would affect the public impression of the government.

Su has been seen as a loyal ally of Tsai, who made him her running mate in their failed 2012 presidential election challenge to former president Ma Ying-jeou.

Following her 2016 election victory, Tsai helped Su become the speakers of the legislature and the two were reported to have closely cooperate in various bills Tsai wanted to push through, including an unpopular pensions bill and last year’s security to increase the penalties for those who work as agents for the mainland Chinese authorities.

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A DPP source who spoke on condition of anonymity said Su’s resignation was a good move as it would save Tsai from being accused of favouritism.

“Some of her enemies, regardless of whether they are from the opposition camp or from within the party, would use the case to attack the president for tolerating corruption, which is what [Tsai] has vowed to eradicate during her stint as president,” the source said.

“It really looks bad especially the president officially took office a little more than 70 days ago,” the source said, adding if the president fails to handle the issue properly, she could face back stabbing from within the party and this would seriously hurt her image and popularity.