LONDON—BP PLC's standby loan has inched up to around $9 billion as more banks join the group of lenders supporting the company against possible claims related to the oil spill in the Gulf of Mexico, a person familiar with the situation said Monday.

Around eight or nine banks have agreed to lend around $1 billion each to BP on a bilateral basis, this person said. "BP is calling on its relationship banks to provide liquidity and more are likely to join the group," this person said.

Among...