My flatmate and I have been sharing a 2 bedroom apartment which also includes a kitchen, one common hall, 2 bathrooms (not attached to any room). I am in another country on a project and therefore we have been sharing this apt. for the last 6 months.
My wife has come down for a vacation for 2 months and is sharing my room.
Before she came down, I informed my flatmate about the same, we decided to divide the food bills, groceries by 3 but decided that the rent will be split into 2 only between the two of us (flatmate and me).
Now, after a month of my wife's stay is over, my flatmate has suddenly asked me to divide the rent also by 3 for the 2nd month.
Is this fair since we had already agreed about the terms of rent division ?
Should I divide the rent for the current month by 3 ? How do I tackle this ?

What justification does the flatmate give for asking to change your contribution from 1/2 to 2/3?

Since this is being done as a verbal agreement outside of the lease, you are in a situation of he said/he said and need to sit down and work this out according to what seems reasonable. Granted your roommate may have agreed to the keep it 2 way split, but 3's a crowd in a 2 bedroom apartment, and most 'living' goes on outside the bedroom. Hogging the fridge and the TV and the couch and whatnot. He probably wasn't aware of what a fifth wheel he would feel like.

My vote (since I don't see this as a real legal question): Agree to the compromise of splitting the second month by 3. I would be grateful that he agreed to have a guest for that long anyway.

I agree with joypulv's logic but you agreed to keep it split by 2 so I'd stick with what you both agreed to. That verbal can not be changed by just one of the parties, you both must agree to the change. However, what is legal and what is fair can be different. We aren't talking big bucks so consider what its worth to you to keep peace for the rest of your stay together

No, I see no justification in this at all.

Thanks everyone for your comments, this really helps!!

His justification is that it is a standard rule to divide the rent by number of people who stay there.
It's strange that this rule has suddenly come up in his rule book because we had discussed this and agreed about the rent payment earlier.

If he had made this clear earlier, I would have moved out for the 2 months and chosen to rent a 1 br place where at least my wife and I have privacy.
I guess more than the money it's his attitude which is causing pain now.