LU managing director Paul Godier responded to speculation over the creation of a "supercompany" by saying: "I would not be sympathetic to a merging of the companies beyond the levels already in place."
John Weight, chairman of Metronet, emphasised that the PPP arrangements encouraged competition but said it was too early to comment on a merger.
Weight said: "Tube Lines has different partners with different backgrounds. That is a good thing. It enables both of us to measure the performance of each other. It is too early to say where it ends."
The consortiums are to share services such as training, but a Tube Lines source emphasised that this did not imply a formal tie-up was envisaged.
The source at Tube Lines, which include Jarvis and Amey, said: "A merged company would be too powerful. We are working closely but neither of us would want a merger. The current arrangement ensures competition."
A merged company would be too powerful. Neither of us want it
Tube Lines source
Metronet, which reached financial close on its two PPP deals last Friday, said this week it would spend £17bn over the next 30 years on nine Underground lines.
Metronet member Balfour Beatty said it expected to carry out £1.2bn of Tube work in the first seven-and-a-half years of the contract. Fellow consortium member atkins will carry out £355m of work in the same period.
Both will oversee modernisation of 150 stations as well as track replacement and other civils work.
A joint-venture management company Trans4Fm has been set up by the two firms. Metronet consortium members Thames Water and Seeboard will manage the work.