In a tournament game, what should a player do if he find that his opponent's position is impossible, which was caused by an earlier illegal move? [duplicate]

Question

For example, if a player realises that his opponent has two bishops of the same colour after a certain legal move (and assuming that it is clearly not caused by any pawn promotion), which implies that his opponent has made at least one illegal move earlier (but it was recognised until much later in the game), what should this player do?

1. If there is a record, can he trace back the illegal move and declare that he wins the game because an illegal move was made earlier?
2. Even without a record, can he still win the game because he can theoretically prove the existence of at least one illegal move made by his opponent?
3. Or should the game continue as it is now, even though the position is impossible?

Answer 1

Section 7.5.1 of the FIDE Laws of Chess has the answer:

7.5.1 An illegal move is completed once the player has pressed his clock. If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. Articles 4.3 and 4.7 apply to the move replacing the illegal move. The game shall then continue from this reinstated position.

The player found guilty is not penalised for the illegal move, provided their opponent has intended to play a move:

4.7.1 A player forfeits his right to claim against his opponent’s violation of Articles 4.1 – 4.7 once the player touches a piece with the intention of moving or capturing it.

Answer 2

If there is a record, can he trace back the illegal move and declare that he wins the game because an illegal move was made earlier?

No. The penalty for a first illegal move is an addition of 2 minutes to the other player's clock. Only a second illegal move loses the game.

According to FIDE Laws of Chess article 7.5.5 -

7.5.5 After the action taken under Article 7.5.1, 7.5.2, 7.5.3 or 7.5.4 for the first completed illegal move by a player, the arbiter shall give two minutes extra time to his opponent; for the second completed illegal move by the same player the arbiter shall declare the game lost by this player. However, the game is drawn if the position is such that the opponent cannot checkmate the player’s king by any possible series of legal moves

.

Even without a record, can he still win the game because he can theoretically prove the existence of at least one illegal move made by his opponent?

No. If no record is being kept of the game then it must normally be blitz or rapid. In that case the illegal move stands and the game continues. According to article A.4.2 of the FIDE Laws of chess -

A.4.2 If the arbiter observes an action taken under Article 7.5.1, 7.5.2, 7.5.3 or 7.5.4, he shall act according to Article 7.5.5, provided the opponent has not made his next move. If the arbiter does not intervene, the opponent is entitled to claim, provided the opponent has not made his next move. If the opponent does not claim and the arbiter does not intervene, the illegal move shall stand and the game shall continue. Once the opponent has made his next move, an illegal move cannot be corrected unless this is agreed by the players without intervention of the arbiter

The illegal move has to be seen immediately (before another move is made or piece touched) and a claim made by player or arbiter.