SEOUL • Ms Kim Dan Bi's brother is the model of the North Korean establishment: An army veteran and member of the ruling Workers' Party, he is now a manager at a state enterprise.

But when he has the time, said Ms Kim, a defector now living in South Korea, her brother helps to trade goods such as television sets and bedding smuggled from China - a sideline lucrative enough for him to have recently bought a car.

"Being in the party doesn't really help financially," she said. "It is even burdensome to those who are running their own businesses."

His story illustrates the challenge posed to ruler Kim Jong Un by the nascent grey market economy that has taken hold in the isolated country as his government prepares for a rare Workers' Party Congress, set to begin on May 6.

For the thousands of delegates who will gather in Pyongyang, attendance at the event will affirm their status among the ruling class.

For a growing number of North Koreans, however, money has overtaken party membership when it comes to getting ahead, according to defectors who have fled to South Korea.

"If you join the party, you lose free time which could be spent selling in the markets because of the party events you're obliged to attend," said a former party member and senior state official who defected to the South in 2014.

"Ordinary people think 'this has got nothing to do with me'," he said, referring to next month's congress. He declined to be identified, in order to protect family members still in North Korea.

Workers' Party culture remains omnipresent in North Korea, where most villages have a building where lectures are delivered by party officials on Saturdays, often to distribute propaganda in areas out of reach for state media.

Party members are also obliged to attend Wednesday lectures after work, said Mr Seo Jae Pyoung, who belonged to the party before leaving North Korea in 2001 and maintains contact with sources inside the country.

The lectures have become more strictly controlled under Mr Kim Jong Un, he said, with a campaign to mobilise people ahead of the congress. Mr Kim used the party's 70th anniversary last year to pledge to introduce "people-first" politics.

Once a regular event, the Workers' Party Congress was last held in 1980. Some Pyongyang watchers see the meeting as a sign that Mr Kim is transforming a country that his father Kim Jong Il ruled through back-channel dealings into a more "normal" state where formal party processes are ingrained.

"The pride people got from being a member of the party has weakened. People only care about money now," said Mr Seo.

No data on party membership is available, although estimates put the number at between three million and four million from a population of 25 million.

Meanwhile, the UN Food and Agriculture Organisation has warned that food production in North Korea has fallen for the first time since 2010 and that it expects worsening hunger as the country struggles with poor rainfall.

REUTERS