The opera singer - who met the Welsh TV presenter on BBC show `Strictly Come Dancing` in 2007 - revealed the pair are "beyond broken hearted" after deciding to call off their engagement but did not give a reason for ending the relationship.
Katherine, 31, wrote on twitter: "Sorry for being quiet of late, its just that I have some very sad news.
"Geth & I have decided to go our separate ways. We are both beyond broken hearted.
"All your hugs & support will be much appreciated xxx (sic)."
Gethin, 33, tweeted: "Hi everybody. I`m deeply sad to tell you that Kath and I have ended our relationship as a couple. We are both heart broken... Gx."
The pair announced their engagement in February 2011 and were due to tie the knot in Wales at some point in 2012.
Katherine broke the news of the split to her mother Susan, 62, earlier this week.
Susan told the BBC: "It is very sad for both of them. It is a difficult tIme and the family will be giving Katherine all the support we can."