text: The staff were hired for the launch of the 24-hour Underground service which was was originally scheduled for September.
  It has been delayed until 2016 due to a dispute over pay and conditions.
  TfL said it is "operationally ready" for the Night Tube and the extra staff are now working across the network.
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  The planned September start date was pushed back until 2016, because TfL and unions were unable to reach an agreement on staffing arrangements.
  Unions took industrial action during the summer having called for increased pay and a limit on how many all-night shifts their members would be asked to do.
  Talks at the conciliatory service Acas are due to resume this week.
  TfL said it is not wasting money as the staff are fully employed.
  It added 307 are in part-time station positions and the rest work in engineering or service control, adding that some of these staff will be used to replace those that leave or retire.
  But Val Shawcross, Labour's transport spokeswoman on the London Assembly, said: "It's a huge amount of money that must be blowing their plans.
  "They've obviously pressed the button too soon in terms of putting them on the payroll."
target: Transport for London (TfL) has recruited 500 staff for the Night Tube at a cost of £1.5m a month, even though the new service is not yet operating.