'We regretfully advise you...' or 'We regret to advise you...' would be correct versions of your example.
The present perfect tense of regret is:I/You/We/They have regretted.He/She/It has regretted.
Genesis chapter 6:6
The past participle is regretted.
Yes, I have regretted cosigning a loan for my ex.
No, I have never regretted skipping arm day at the gym.
Yes, I have regretted cosigning a car loan for my ex.
The cast of Deeply Regretted By - 1978 includes: Donal Farmer as Priest
Regretted.
no
Yes all of his tattoos are permanent and yes there real. In an interview or something he did say he regretted a couple of his tattoos
Regretted
He regretted his choice