US carriers Delta and Northwest Airlines are tipped to unveil details of their long-delayed merger by tomorrow.

The deal could value Northwest at $3 billion (£1.52 billion) well below its market value of $4.6 billion just over two months ago.

The declining value reflects the tough times for airlines in the US with rising jet fuel costs and a softening economy.

One factor that may hold up a deal is finding agreement with the airlines' 6000 pilots.

Delta was locked in talks with union representatives over the weekend over a new post-merger contract.

Northwest's 5000 pilots say they will evaluate whether the carrier has better prospects as an independent.

The group decided any merger, "in order to avoid our vigorous opposition, must clearly be in the interests of NWA pilots, customers and employees".

Both Delta and Northwest, have recently said they plan to reduce domestic capacity and retire planes.

Delta also announced plans last month to cut 2000 jobs, which represents about 4% of its workforce.