The highest-profile North Korean defector in the South declared himself a candidate for parliament Tuesday, in a move he said would demonstrate democratic freedoms in his new home.

Thae Yong Ho, who fled his post as the North's deputy ambassador to Britain in August 2016, has since become a prominent and outspoken critic of Pyongyang and the engagement approach pursued by the South's President Moon Jae-in.

He had joined the main opposition Liberty Korea Party (LKP), he said, adding his victory would encourage Northerners.

South Korean media reports cited LKP officials saying he would be recommended for a constituency in Seoul's wealthy Gangnam district, a party stronghold, giving him a strong chance of success in the April 15 legislative election.

"Once the North Korean people and elites see that Thae Yong Ho, who served as a North Korean diplomat, can be elected by South Koreans," Thae said, "we will be a step closer to the real reunification that we hope for".

His impoverished but nuclear-armed former homeland is subject to multiple sanctions over its weapons programmes and remains technically at war with the South.

Moon's dovish policies on North Korea and reunification were going in the "wrong direction", Thae said.

Thae Yong Ho fled his post as North Korea's deputy ambassador to Britain in August 2016
Thae Yong Ho fled his post as North Korea's deputy ambassador to Britain in August 2016 YONHAP / -

Moon brokered the first summit between US President Donald Trump and North Korean leader Kim Jong Un, but since the collapse of a second summit between the two in Hanoi he has found himself largely sidelined by Pyongyang.

"I know about the North Korean regime and system more extensively and more deeply than anyone else," Thae said, adding he would offer a "realistic" approach that differs from "unconditional assistance or unconditional confrontation".

Thae is one of the highest-ranking North Korean diplomats ever to defect to the South, gifting Seoul's then-conservative government a major propaganda coup at a time of rising tension on the divided Korean peninsula.

If elected, he will become the first former North Korean official to win a seat in the South's National Assembly.

A predecessor of the Liberty Korea Party had a defector lawmaker from 2012 to 2016, but Cho Myung-chul was an academic and did not serve in the North Korean government.

About 33,000 North Koreans have fled to the South in the past two decades, but it is rare for high-level officials to defect.

Following his arrival in Seoul, Thae told reporters that he had decided to defect after becoming disillusioned with the Pyongyang regime and to avoid his children from living "miserable" lives in the North.

The North's state media denounced him as "human scum" and accused him of embezzling state funds, raping a minor and spying for South Korea in exchange for money.