The RMT rail union on Tuesday raised the alarm after it emerged that Tube Lines, the private underground maintenance and infrastructure company, faced a potential £2bn ($3.5bn) funding deficit for the next phase of its upgrade contract on the network.

Bob Crow, general secretary of the RMT, said the situation was reminiscent of the financing shortfall that led to the collapse last year of Metronet, which was taken over by Transport for London.