Shares in Grab, Southeast Asia's biggest ride-hailing and delivery firm, fell 9% in their Nasdaq debut following its record $40 billion merger with a special purpose acquisition company.

The backdoor listing on Nasdaq marks the high point for the nine-year-old Singapore company that began as a ride-hailing app and now operates across 465 cities in eight countries, offering food deliveries, payments, insurance and investment products.

Grab's shares rose as much as 21% minutes after the listing before retreating to trade 9.3% lower at $9.99 by midday in New York.

"The price makes no difference to me. I'm going to celebrate tonight and get back to work tomorrow," Chief Executive Anthony Tan told Reuters just after the shares started trading.

Grab kicked off the biggest US listing by a Southeast Asian company with a bell-ringing event in Singapore, hosted by Nasdaq and Grab's executives.

The event was attended by about 250 people including its investors, drivers, merchants and employees, with many dressed in the company's signature green.

Grab's flotation "will provide a bigger cash buffer" to its "cash burn", S&P Global Ratings said in a note.

But it said the company's "credit quality continues to be constrained by its loss-making operations, and free operating cash flows could be negative over the next 12 months."

Southeast Asia's internet economy is forecast to double to $360 billion in gross merchandise value by 2025.

Bonanza for backers

CEO Tan, 39, expanded Grab into a regional operation with a range of services, after launching it as a taxi app in Malaysia in 2012.

It later moved its headquarters to Singapore.

He will control 60.4% voting rights along with Grab's co-founder, and president Ming Maa, but hold only a 3.3% stake with them.

Grab's listing brings a payday bonanza to early backers such as Japan's SoftBank and Chinese ride-hailing giant Didi Chuxing, which invested as early as 2014.

They were later joined by the likes of Toyota Motor Corp, Microsoft Corp and Japanese megabank MUFG.

Uber became a Grab shareholder in 2018 after selling its Southeast Asian business to Grab following a five-year battle.