STOCKHOLM, Nov 30 (Reuters) - A Swedish court on Wednesday reduced a fine imposed on Nasdaq Clearing by the country's financial watchdog in 2021 for deficiencies uncovered following the default in 2018 of a Norwegian power market trader.

The Stockholm Administrative Court of Appeal said in a statement it had cut the fine to 250 million Swedish crowns ($23.7 million) from 300 million after Nasdaq Clearing appealed the Financial Supervisory Authority's decision.

Clearing houses like Nasdaq's are vital for the stability of markets, acting as an intermediary in stock, bond or derivatives transactions and ensuring completion if one side goes bust.

The private trader's default in 2018 following big fluctuations in power market spreads forced other members of the clearing house to pay 107 million euros ($110.8 million) to restore contingency funds.

The FSA had warned and fined Nasdaq Clearing for insufficient follow-up of memberships, miscalculations of trading margins and over its risk management.

The appeals court said that while Nasdaq Clearing had violated guidelines, the size of the original fine was disproportionate. ($1 = 10.5423 Swedish crowns) ($1 = 0.9660 euros) (Reporting by Anna Ringstrom, editing by Terje Solsvik and Jane Merriman)