Balloting has begun this week for two separate disputes on the London Underground, ahead of possible strike action.

Around 4,000 members of station staff are being balloted by the Rail, Maritime and Transport (RMT) union over the impact on safety from the axing of nearly 900 front line workers alongside the closure of Tube ticket offices. RMT says the cuts have "left safety on a knife edge" with the incidents at Canning Town and North Greenwich throwing the spotlight on dangers which are being worsened by the cuts process.

Read more: Here are all the rail and station closures in London over Christmas

Mick Cash, RMT's general secretary, said: "With the constant overcrowding on stations and platforms it is only a matter of time before there is a major tragedy if we don’t act decisively. Our dispute is about taking action to haul back the cuts machine and put safety back at the top of the agenda."

Mark Wild, London Underground’s managing director, said: “The safety of customers and staff is our top priority and our hardworking people ensure that London Underground remains one of the safest metros in the world. There are more staff available to customers in public areas of stations than ever before and crime is at its lowest ever level. An independent review‎ of the closure of ticket offices is being conducted by London TravelWatch to ensure that we continue to deliver high quality customer service."

North Greenwich station was evacuated on Thursday 20 October after a suspicious item was found on a train. British Transport Police later said "a controlled detonation" took place to ensure the item was safe. A 19-year-old man was later charged of making or possessing an explosive substance with intent to endanger life.

Read more: More Southern strikes are going ahead this week

Separately, Piccadilly line workers are being balloted over breaches of policies, procedures and safety which the union says amounts to a wholesale breakdown in industrial relations. Voting closes on Tuesday 15 November.

Cash said safety was again "a major factor" in the balloting of drivers on the Piccadilly line. He said: "Our members have been left exposed and vulnerable and we have no choice but to blow the whistle before lasting damage is done."

Brian Woodhead, London Underground Operations Director, said: “We urge the RMT to work with us constructively to find solutions to the issues they have raised, rather than disrupting Londoners with strikes. We remain keen to engage with the trade unions and are open for talks at ACAS in order to resolve this dispute as soon as possible.”

On 4 and 5 November, more strikes are going ahead on Southern Rail. The RMT and Aslef are balloting train drivers about more possible strikes.