Clifford Chance lawyers are in negotiation with bosses over getting extra cash if they are stuck working on deals as the new millennium is rung in.
A spokesman for the firm confirms that people will be in the office at midnight on 31 December IT staff as well as lawyers, if required.
"If that's what the client wants, that's what the client will get," says Tom Rose, Clifford Chance's head of marketing. "It will be the usual type of business working on deals. The numbers involved are unquantifiable."
He says the question of whether lawyers will be paid extra for working on new year's eve is a "big issue" and one which is being reviewed.
But he declines to say whether lawyers will face the sack if they refuse to work that night: "We would take on board the concerns of individual solicitors, consider them and address the issue."
Other big City firms confirm they will be open for business, but say no decision has been made on bonus payments.
A Slaughter and May spokeswoman says: "It's something we're considering at the moment, who isn't? We just haven't got as far as working out the details yet."
A Freshfields spokeswoman says: "Basically we will be working." She declined to say whether extra payments will be made.