Both private consortia responsible for maintaining the network, Tube Lines and Metronet, claim long-term closures are the only alternative to recurring delays when night engineering work overruns. A London Underground spokesman said: "We are investigating long-term closures of sections of the Northern line." He warned of "complex negotiations" with Tube Lines before LU would agree to the closures. If it does, shutdowns would probably begin next year, each lasting four to eight weeks.