The head of Credit Suisse, Tidjane Thiam, said Friday he would step down next week, as a ballooning spying scandal rocks Switzerland's second largest bank.

Thiam's position appeared untenable after revelations that the bank had spied on its employees as well as a former top executive, and allegation it was also keeping tabs on Greenpeace.

"I have agreed with the Board that I will step down from my role as CEO," Thiam said in the statement.

Credit Suisse said its board had "unanimously accepted" Thiam's resignation, which would take effect on February 14, a day after the presentation of the bank's fourth quarter and full-year results for 2019.

He will be replaced by the current head of the bank's Swiss operations Thomas Gottstein, it said.

Thiam, a 57-year-old French-Ivory Coast national, previously held at the top post at British insurer Prudential before taking the helm of Credit Suisse five years ago.

Bank chairman Urs Rohner said the outgoing chief executive had "made an enormous contribution to Credit Suisse".

Private investigators

But Thiam, who once served as an Ivorian government minister, had become increasingly embattled amid a swelling scandal over revelations the bank had spied on employees.

The bank has also faced allegations it was keeping tabs on environmental activists angered by its oil and gas investments.

The scandal exploded last September, with the discovery that surveillance had been ordered on star banker and former wealth management chief Iqbal Khan.

Khan was tailed after he jumped ship to competitor UBS, sparking fears he was preparing to poach employees and clients.

The spying was revealed after Khan confronted the private investigators tailing him, leading to a fight in the heart of Zurich, and ending with Khan pressing charges.

An initial investigation blamed former chief operating officer Pierre-Olivier Bouee, who stepped down in October, but found no indication the rest of the top brass, including Thiam, were involved or even aware of the spying activities.

Then in December, an internal Credit Suisse probe confirmed that a second executive had been spied on by the bank, again blaming Bouee alone for incident.

'No knowledge'

"I had no knowledge of the observation of two former colleagues," Thiam insisted in Friday's statement.

"It undoubtedly disturbed Credit Suisse and caused anxiety and hurt. I regret that this happened and it should never have taken place," he added.

The scandal broadened last weekend, when a Swiss weekly reported that the bank had also ordered surveillance on Greenpeace, including accessing militants' emails, to try to find out when they were planning demonstrations against the bank.

In 2017, Greenpeace gatecrashed the bank's annual general meeting, with demonstrators unfurling a banner to protest links with companies behind a controversial oil pipeline in the United States.

Credit Suisse has also recently been the target of climate campaigners, including Greta Thunberg, who urged Swiss tennis great Roger Federer to cut ties with the bank over its fossil fuel investments.

Gottstein, who will take over the helm of Credit Suisse next week, has worked for the bank for more than two decades, and currently heads its Swiss operations and is a member of the executive board, the bank statement said.

"Based on his deep and comprehensive experience in our business and in view of his impressive performance as head of our Swiss bank and his respect amongst our clients and employees, Thomas Gottstein is excellently positioned to lead Credit Suisse into the future," company chairman Urs Rohner said in the statement.

Following the news, Credit Suisse saw its share price plunge 3.56 percent to 12.34 Swiss francs a piece in morning trading, as the Swiss stock exchange's main SMI index slipped just 0.23 percent.