Dilemma , honest opinion needed, as if your own life depends on it.?

You made a promise > keep it and people will suffer.

Break the promise and who suffers? > Your relationship with the person you promised.  Or maybe yourself.

The problem with your question is the qualification of "high probability".

Will people suffer or won't they?

Which choice produces the greatest good?

Put yourself in the place of those many people who would suffer.

... then ask yourself the same question.

What is the right thing to do? 

Usually the right thing is also the hardest.

The Virgin Mary is Our Lady of Good Counsel. She can advise one with any problem in life. It is good to honor her daily by reciting her rosary with care.

Here's some guidance from the Bible.

Ex 1:15 The king of Egypt said to the Hebrew midwives, whose names were Shiphrah and Puah, 16 “When you are helping the Hebrew women during childbirth on the delivery stool, if you see that the baby is a boy, kill him; but if it is a girl, let her live.” 17 The midwives, however, feared God and did not do what the king of Egypt had told them to do; they let the boys live. 18 Then the king of Egypt summoned the midwives and asked them, “Why have you done this? Why have you let the boys live?”19 The midwives answered Pharaoh, “Hebrew women are not like Egyptian women; they are vigorous and give birth before the midwives arrive.”20 So God was kind to the midwives and the people increased and became even more numerous. 21 And because the midwives feared God, he gave them families of their own.

The Hebrew midwives lied to Pharaoh in order to save lives, and God did not hold that against them.  Don't get me wrong, lying will send people to hell (Rev 21:8).  But if your situation is similar to what the midwives encountered, maybe you should lie about the promise you made.

It really depends on what the consequences are - is the person going to do a terrorist attack?  I'd break that confidence in NY minute.  If it's of a personal nature (not life threatening) I'd keep their secret.  If it bothered me so much and I couldn't live with it, I'd talk to the person and ask them to release me from my promise - if they didn't release me from it - I'd keep my mouth shut.  

In my opinion, this should be in Polls.

I would go BACK to the person I made a promise to (if they are still alive), and tell them that I couldn't honor my promise to them.

can you write it once more, but this time slower?