The opera singer took to Twitter to confirm her happy news as speculation over whether the pair had taken their relationship to the next level began to mount.
Shortly after posting on the site, Katherine received plenty of congratulatory messages from her celeb pals including Myleene Klass and Piers Morgan.
According to the Daily Mail, Katherine and Andrew will marry in 2015, with ceremonies in Wales and his native US.
“Katherine has been preparing the announcement for a while. She is very excited – she loves how artistic and intelligent Andrew is,” a pal reportedly told the paper. “He is considering moving to the UK for her.”
According to sources, the couple could be planning to add to their family shortly after tying the knot.
“She has definitely left her party ways behind her since meeting Andrew, and that means some of her socialite friends have been left feeling as if they have been discarded,” another 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 was previously engaged to presenter Gethin Jones, however they split in 2011.
She was awarded an OBE in March for her charity efforts and musical talents.
The opera singer’s prolific music career has included seven Classical Album Chart number ones and she’s made continued efforts to raise cash for charities, and ran the London Marathon in 2013, raising over £25,000 for Macmillan Cancer Support, in memory of her father.