RMT plans a 24-hour stoppage on December 31 and another on January 8/9 following a staffing dispute.

The strike could cause travel chaos on one of the busiest nights of the year and prevent people from taking advantage of the free Underground service on New Year's Eve which had been planned to run from 11:45 pm to 4:30 am on New Year's Day.

A spokesman for London Underground told reporters they would try again next week to hold more talks with the union at the conciliatory service ACAS.

The first strike is set to begin at midday on New Year's Eve while the second starts at 6:30 pm on January 8.

RMT have said front-line station staff, who were praised for their work on July 7 when three suicide bombers caused carnage on the Underground, were being moved to different roles which could hamper safety levels.