The posters were created by the Voluntary Agency Network of Korea (VANK) and reportedly placed on the Japanese embassy in Seoul last month.

The poster also includes an image of radioactive material coming from the Olympic torch.

Tokyo 2020 has been billed as the “Reconstruction Olympics” by Japanese authorities, with the Games said to be highlighting the country’s recovery from the 2011 earthquake and tsunami.

Around 16,000 people lost their lives in the tragedy.

The natural disaster also led to an accident at a nuclear power plant in Fukushima.

The Torch Relay for the Games is due to begin in the Fukushima Prefecture, with both Tokyo 2020 and the International Olympic Committee (IOC) keen to use the Games to aid the region's recovery.

In December Greenpeace warned that radiation hot spots had been detected near to the starting point of the Torch Relay in Fukushima, although Japanese authorities say the area is safe.

VANK claim their protesters were aimed at highlighting concerns over radiation.

“We included messages of warning about the safety of radiation, the biggest concern during the Tokyo Olympics,” the group said.

“Host country Japan said agricultural products from Fukushima Prefecture are safe and announced that it will provide them for Olympic athletes.”

Hashimoto responded to the posters by claiming the actions were “very regrettable”, with the Japanese authorities having reportedly raised the issue with the South Korean Government.

“The use of such a poster is unacceptable,” he said, according to the Japan Times.

“The reality is totally different,” said Yoshihide Suga, Japan’s chief cabinet secretary, according to the Asahi Shimbun.

“We can never overlook the issue.”

VANK, the group responsible for the posts, was founded in 1999 with the aim to “properly convey South Korea to the world through the internet.”

The group has received criticism that their promotion of South Korea has come at the expense of Japan.

VANK are among the groups to have opposed Japan’s use of the Rising Sun flag.

Japan have refused to ban the Rising Sun flag at Tokyo 2020, with the decision having prompted criticism from both North and South Korea.

South Korea's Sports Ministry wrote to the International Olympic Committee (IOC) to express its "deep disappointment and concerns" about the use of the flag, viewed by some overseas as a symbol of Japan's aggression leading up to, and during, the Second World War.

The Sports Ministry in Seoul reportedly complained to the IOC that the flag should be compared to the swastika symbol used in Nazi Germany.

They said the Rising Sun Flag was used in Japan by "extreme right-wing organisations" and in "xenophobic demonstrations".

Japan conquered large parts of Asia, including the entire Korean Peninsula, before their surrender in 1945.

Relations between the countries have remained tense since, with the relationship having deteriorated further in recent months.

South Korea last year threatened to review plans to hold a pre-Olympic Games training camp in Japan next year because of concerns relating to radioactivity from the 2011 Fukushima nuclear disaster.

Most observers, however, believed South Korea's threat was linked to the deteriorating political relations between the two countries.