The 'Get Over You' star and husband Richard Jones of The Feeling are already parents to Sonny, 7, and Kit, 2.
Ellis-Bextor confirmed the news on Twitter this afternoon, writing: "I am having another baby. Due in Spring. A little brother or sister for Sonny and Kit. Family e-b-jones are happy and excited. It's a lovely thing."
The singer also reassured fans that she was continuing to record material for her fifth studio album.
"By the way, work on album 5 continues as usual," she said. "In fact, I now have a deadline! In the studio as we speak."
The 32-year-old released fourth album Make A Scene during the summer. The album peaked at number 33 in the UK charts.
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Watch Sophie Ellis-Bextor's 'Get Over You' video below: