(Reuters) -Singapore Airlines reported a slump in half-year earnings on Friday, reflecting stiff competition and higher costs, but added that it will post a gain of S$1.1 billion ($832.45 million) once the Air India-Vistara merger is completed.

The city-state's flag carrier is expecting the merger between Tata Group's Air India and Vistara, a joint venture between Singapore Airlines and Tata, to be completed by the end of November.

The airline will recognise a non-cash accounting gain post the completion.

Singapore Airlines (SIA), which is set to get a 25.1% stake in Air India, will inject S$498 million into the new combined entity, it said on Friday.

The island nation's leading airline operator posted a 48.5% plunge in net profit to S$742 million for the April-September period and declared an interim dividend of 10 Singapore cents per share.

"Increased capacity and stronger competition in key markets led to yield moderation, resulting in lower operating profit," Singapore Airlines said.

Airlines globally have ramped up the number of flights and routes to cater to robust air travel demand, which has resulted in increased competition, putting pressure on ticket prices and squeezing profit margins.

SIA's total expenses rose to S$8.70 billion for the half-year ended Sept. 30, a 14.4% rise, due to a jump in both fuel and non-fuel costs.

Revenue rose by 3.7% from a year ago to S$9.50 billion.

Passenger load factor - a measure of how many seats are filled on planes - was 86.4% in the first half for the group as a whole, compared with 88.8% a year earlier.

($1 = 1.3214 Singapore dollars)