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The Night Tube today suffered its first strike threat – just six months after 24-hour operation began.

The RMT ordered a strike ballot among its Night Tube drivers on the Central, Victoria, Northern, Jubilee and Piccadilly lines with a result to be declared on 14 March – meaning walkouts could take place towards the end of next month.

Union leaders ordered the ballot in a complex dispute over working conditions and overtime pay. Driver salaries start at just under £50,000 but can rise to £60,000 after five years.

Mick Cash, the RMT leader, said the Night Tube agreement which is different from that covering other drivers, is “blatantly discriminatory” as it prevents promotion and overtime pay does not apply until after the driver has exceeded 35 hours in a week.

This, he said, prevents overtime being paid if there is late finish to the service because of earlier disruption.

Night Tube began last August on the Central and Victoria lines, nearly a year later than planned because Tube and union bosses could not reach an agreement on the complex shift arrangements, pay and working conditions.

Mr Cash said: “It is outrageous that London Underground (LU) have decided to discriminate against their Night Tube drivers.

“This is a senseless and damaging policy that picks out one group of staff for negative treatment and of course the drivers are angry and that is why we are balloting for action.

“RMT would call on LU to see sense, stop these stupid attacks on career progression and fair reward for overtime working and confirm that this discrimination against the Night Tube drivers has been lifted.”

The union is understood to represent just under half of Night Tube drivers with the remainder mostly belonging to the rival Aslef union.

A walkout by RMT members would cause huge disruption to the 24-hour service – but some Aslef drivers are also said to be unhappy with the situation and the union could join in the dispute.

A second strike by RMT drivers on the busy Central line, used by more than 800,000 passengers a day, is due to begin from 9 pm tomorrow and lasting all day Wednesday.

LU warned the Waterloo & City line, which uses the same pool of drivers, is not expected to operate at all on Wednesday.

The dispute is over the “forced” transfer of eight drivers from their home depots in the East of London to those to the west.

The first 24-hour walkout last month caused huge disruption.

LU said the Central line would close from 11.30 pm tomorrow.

On Wednesday there will be no Waterloo & City and no Central line east of Leytonstone; from 05:30 there will be no westbound Central line between White City and Leytonstone

From 7 am there will only be a “very limited” service throughout the entire line. It will close at 11 pm.

Some other stations and lines will also be much busier during the strike, particularly in the morning and evening peaks. These include Chingford, Ilford and Leytonstone.

Other stations and lines that will also be much busier include TfL Rail services between Romford and Stratford, particularly Ilford station, and London Overground lines and stations between Liverpool Street and Chingford.

The Victoria line will be much busier than usual, particularly Walthamstow Central, as will the Jubilee line between London Bridge and Waterloo.

Customers may have to queue outside stations during peak times to prevent overcrowding, and it may take longer than usual to board a train.