Adyen, which processes payments for online merchants including Facebook and Netflix, said that founders CEO Pieter van der Does and chief technology officer Arnout Schuijff were each selling 15% of their stakes in the company. The shares to be sold combined are worth around 320 million euros.

"The reason for the sale is to diversify their portfolios and reduce single stock risk," Adyen said in a statement published after the stock market closed.

"Both Pieter and Arnout remain committed to Adyen in their current roles ... this sale in no way reflects a change in their views on the company."

Adyen's shares were down around 5% on Thursday even though the group had earlier reported a better-than-expected 79% jump in core profits.

First-half core earnings (EBITDA) at Adyen, one of Europe's better-known fintech firms after its strong market debut in June 2018, reached 125.8 million euros (£114.8 million), up from 70.3 million a year before.

The shares doubled in value on their debut and hit a peak of 758.9 euros in September from an issue price of 240 euros.

CEO Van der Does said growth was driven by a higher volume of transactions handled for existing customers.

The company grew strongly on all key metrics, with net revenue up 41% to 222.1 million euros and net profit up 92%. It processed 104.6 billion euros worth of payments, up 49% from 70 billion in the first half of 2018.

Analyst Johann Scholtz at Morningstar, who recently initiated coverage of Adyen with a "fair value" estimate of 425 euros per share, said there was no question the company's growth was impressive and noted it had also reduced costs in the quarter.

"Even with robust estimates we can't get to current market valuations - there's a lot of good news priced into the share," Scholtz said.

Thursday's share price dip on the back of 40%-plus revenue growth "illustrates expectations are quite high," Scholtz added.

Adyen shares closed down 5% at 643.8 euros.

Adyen's pitch to its merchant customers is that it is able to juggle nearly any kind of payment, routing shoppers through its single platform with fewer errors than competitors.

The company said it had added new customers including North Face and Timberland. It also added new payments methods to its platform, including local deals with Apple and Google Pay, as well as Open Banking in Britain and M-Pesa in Kenya.

CEO Van der Does said: "What we do see is a healthy flow of new merchants from the pipeline converting into customers."

"If you look at the size of the pipeline, we're confident this process will continue."

(Reporting by Toby Sterling; Editing by Rashmi Aich/David Holmes/ Alexandra Hudson/Jane Merriman)

By Toby Sterling