ATLANTA — An impasse among pilot negotiators over determining seniority put a $20 billion deal to combine Delta Air Lines Inc. and Northwest Airlines Corp. in “serious jeopardy” as the companies’ boards prepared to meet Wednesday, two people close to the talks told The Associated Press.

The people said the pilots unions have agreed on a comprehensive joint contract, but they are unable to agree to how seniority for the 12,000 pilots would work under a combined carrier. The people asked not to be named because of the sensitive stage of the talks.

They said late Tuesday that the pilot talks were expected to continue Wednesday, but if no agreement is reached, a deal on a combination of the two airlines would be in real trouble.

The boards of both companies were expected to vote on a combination agreement Wednesday if a pilot deal is in place by then. Otherwise, they were expected to just get an update on the merger talks, three people close to the talks said.

One of the officials close to the talks said Northwest’s board might only meet by teleconference or, if things fall apart, not meet at all.

A Delta spokeswoman declined to comment on consolidation issues involving the carrier. Delta has previously said it was considering a possible consolidation transaction, but has not commented beyond that.

Greg Rizzuto, a spokesman for the Northwest pilots union, said Wednesday that while most of the economic and contractual issues between the unions have been worked out, protecting the seniority of Northwest pilots remains a problem.