London Underground's biggest contractor has hit back at criticism from the capital's mayor over delays. Tube Lines, which is owned by Ferrovial and Bechtel Corp, said work to overhaul the Jubilee line would be completed by 31 December – but only if it was allowed to close the line for five extra days to give crews access to the tunnels. The firm has a 30-year contract to upgrade key parts of the Tube network, but mayor, Boris Johnson, has expressed concerns over when work will be completed. He said: ‘I am hugely frustrated at the progress to date, which is simply unacceptable.' Fears had already been heightened over the future of London Underground's PPP deals, after the collapse of Metronet. The two sides will now meet to resolve the problems. A Tube Lines spokesman said: ‘We are asking for just five more days of access over the next four months. We could then deliver, on time, the benefits to passengers.'