Let me explain my problem
I got promotion in 1993 on the basis of my joining date seniority. But on that time I was on leave. After one month I joined in my earlier and given the acceptance letter on that day onwardi got the financial benefit. Now my colleagues got promotion and I have been deprived by saying that I had lost my seniority by late joining the last promotional post.
Is it true? May I loose my seniorty?

This is a question that ONLY your employer can answer. There is no World Law about seniority, nor even by country.

You don't explain 'on leave.' The only 'leaves' I know about are in academia and the military.
I can't tell if you lost seniority by taking an extended time off from work that was not vacation time, or because you failed to begin your new duties after the promotion.

In any case, an employer SHOULD have written policy. Ask them. And maybe start looking around at other job options.

A Joypulv mentioned... In the Union world (in the USA anyway)... if you are on UNPAID leave.. you don't accrue seniority or benefits during that time. It's the same here in the Non-union world too.

Also unless you have a Union contract specifying so by negotiated agreement... then you aren't ENTITLED to pay raises OR promotions... IF, WHEN, AND HOW MUCH, are completely between you and your employer and always negotiable. Most cases if you feel you deserve more than they are willing to give... you look for a better job elsewhere.

Your employer is not obligated to always use just seniority for promotion, he can use any criteria he wants like better performance or qualifications. Even in union shops this can happen. If you have a union, consult with them. If you don't, the HR can clarify this.

As was said there should/could be a written policy on this matter of seniority rights for you to review for yourself. There is almost no such thing as automatic promotion based on seniority at a company deemed management level.