Hidden camera footage apparently showing South Korea's first lady accepting a luxury designer handbag has engulfed President Yoon Suk Yeol in controversy, threatening his party's April election prospects.

Dubbed the "Dior bag scandal" by local papers, it has triggered a slump in Yoon's already low approval ratings and set off a firestorm in his party, just months out from the key polls where they hope to win back a parliamentary majority.

AFP takes a look at what we know:

What happened?

A left-leaning pastor, who disapproves of Yoon's hawkish North Korea policy and claims to be concerned about the first lady's influence on the administration, used a wristwatch to record himself presenting Kim Keon Hee with a $2,200 designer handbag.

The first lady, a self-professed animal lover best known internationally for her work pushing South Korea to ban dog meat, is heard telling the pastor: "Please don't buy an expensive item like this".

But she is not seen returning the bag in the footage, first released last year by a left-wing YouTube channel critical of Yoon. The pastor has also claimed he presented her with expensive cosmetic items as a gift at a previous meeting.

The gift would violate South Korean law banning public officials and their spouses from accepting gifts worth more than $750.

The president's office declined an AFP request for comment.

An official from Yoon's office has told the Yonhap News Agency that the gift was being "stored according to the relevant regulations".

Why now?

For weeks, the ruling party ignored the bag incident, and neither the president nor his wife have addressed it publicly.

But a member of Yoon's party, Kim Kyung-yul, last month likened the situation to Marie Antoinette, the French queen known for her lavish lifestyle.

Local media reports speculated that Yoon was livid and wanted to remove the party's leader over it, even as other lawmakers lined up to defend the first lady, decrying a "smear campaign" and blaming the pastor's "foul intentions".

Why did the pastor do it?

Pastor Choi Jae-young, who is in favor of better ties with Pyongyang and has visited the North many times in the past, says he decided to film his meeting with the first lady after becoming concerned about her power within the president's office.

He claimed at a press conference to have seen Kim direct the appointment of a senior finance official in a phone conversation during an earlier meeting.

"I was appalled from seeing Kim exercising power as if she were in charge of official appointments," Choi said.

The YouTube channel filmed the entire purchase of the Dior bag at a luxury store in Seoul, to demonstrate how much it cost, before the pastor presents it to Kim.

Prosecutors have launched an investigation into Choi, local reports said, after he was sued for breaking and entering and obstruction of justice by a civic group.

Was it fair?

While secret filming was used in "trapping" the first lady, she "received an item she should not have received", a column in the widely read Dong-A Ilbo newspaper said Tuesday.

"I wonder whether there has ever been as much public interest in a Dior bag before," the column said.

It is not the first time Kim has faced public scrutiny. During Yoon's presidential campaign, she was forced to apologise over falsified credentials.

She has also faced accusations of stock manipulation, with the opposition-controlled legislature passing a special bill mandating a probe that was later vetoed by her husband.

But those claims have failed to capture the public's imagination like the handbag video, attorney and political commentator Yoo Jung-hoon told AFP, likely because "stock manipulation by nature involves complex legal and technical terms" not as easily grasped.

"Everyone knows Dior and there's even direct footage of the meeting, making it very straightforward for the public to digest," he said.

And the election?

The "Dior bag scandal" has hit Yoon's already low approval ratings, which analysts say could hurt his party's prospects in April.

About 70 percent of people believe he needs to address the issue, according to a recent survey by local broadcaster YTN.

The incident could have been contained if Yoon had explained it earlier, but it could now "chip away both at Yoon's and his party's approval ahead of the April elections", commentator Yoo said.

The first lady herself has not spoken publicly for over a month, even when lawmakers approved in early January a dog meat ban -- a significant PR victory for her, after her long campaign.

In South Korean culture, "there's a great emphasis for public officials to separate their official conduct from personal matters", said Cho Ki-suk, professor of international studies at Ewha Womans University.

"It's hard to see this Dior bag controversy as a gender issue. I think Yoon will apologise over this in the end, because without his apology, there isn't much his party can do in election campaigning to win."