The classical singer, 33, has been dating 36-year-old Andrew since last October, who is said to have proposed to her sometime last week.
The pair managed to keep their relationship under wraps until last month, when Andrew was at her side when she collected her OBE at Buckingham Palace.
Speaking to a UK magazine, a source close to the singer said: “Katherine has been preparing the announcement for a while. She is very excited – she loves how artistic and intelligent Andrew is. He is considering moving to the UK for her.”
Katherine currently spends the majority of her time in New York with Andrew, where he is a professor at the New York University.
Although Katherine and Andrew are yet to confirm, it’s reported that they will marry next year. It is thought they will have two ceremonies – one in Wales with all of Katherine’s friends and family, and one in New York.
A friend told the paper: “This relationship is moving very fast – they are so in love. I wouldn’t be surprised if she is expecting by this time next year.”
Katherine’s last high profile relationship was with presenter Gethin Jones, who she got engaged to in 2011. However the couple split later that year.
She took to Twitter this weekend to thank fans and friends for their congratulations.
She posted: “Thank you for all your lovely wishes of congratulations. They mean so much to us. Happy Easter!”
Myleene Klass was one of the celebrities to send her wishes, writing: “Amazing news. Very happy for you x”