North Korea hacked into the personal emails of an aide to the South Korean president, his office has confirmed to the OceanNewsUK.
The breach occurred in the run-up to President Yoon Suk Yeol's state visit to the UK last November.
The staff member was hacked after using a personal email account for official work, the president's office said.
A local newspaper cited a high-ranking government source as saying hackers had accessed Mr Yoon's trip schedule.
The Kukmin Ilbo newspaper added that messages sent by the president had also been stolen.
But the president's office would not divulge what information was stolen.
This is thought to be the first time the North has successfully hacked a member of the South Korean president's team.
In a statement, the president's office stressed that its security system had not been hacked.
"The breach was caused by a careless violation of security regulations by an individual administrator who used a commercial email for work purposes," it told the OceanNewsUK.
North Korea uses cyber hacking to steal both money and information, and its means are becoming increasingly sophisticated.
Pyongyang is subject to extreme international sanctions, and its cyberhackers seek to steal large sums of money, often in cryptocurrency, to fund the regime and its nuclear weapons programme.
It is estimated to have stolen as much as $3bn since 2016.
North Korea is also thought to carry out hacks with the purpose of stealing state secrets, including details of advanced weapons technology.
The South Korean government source, who spoke to the Kukmin Ilbo on the condition of anonymity, said they were "beyond shocked and appalled" when they learnt about the breach, adding it could have caused problems for Mr Yoon's security abroad.
President Yoon visited London for a three-day state visit in November, during which he met King Charles and Queen Camilla, as well as Prime Minister Rishi Sunak.
The South Korean government said that the incident had been detected before the president's trip began, and the necessary measures were taken to address it.
It added it had taken steps to strengthen its security, including raising awareness among its team, to prevent another such incident from occurring.
