The Rail, Maritime and Transport has set a deadline for a row over ticket offices to be settle
The Rail, Maritime and Transport (RMT) union said jobs were threatened under plans by Greater Anglia to close a number of ticket offices.
The union has given the company a deadline of June 19 to confirm that no jobs will be lost and all windows will be kept open for the length of the current franchise until 2026.
The RMT is also seeking confirmation from Greater Anglia over the retention of the current two-person train despatch procedures.
The union said it had been told that the current 64 offices will be cut to just seven, although the company denied this.
RMT general secretary Mick Cash said: "The union is appalled at this planned attack on ticket offices and station staff across the Greater Anglia franchise, which would wipe out the vast bulk of these vital passenger services over the next year.
"The union will fight with every tool at our disposal to halt these disastrous plans, which are all about cutting public services to pump up private profits.
If Greater Anglia fail to meet the deadline ta formal dispute will be declared
"We have given the company a clear deadline to give us assurances on both ticket office and train despatch issues and the ball is now in their court.
"If we don't get a positive response by that date, we will have no option but to declare a formal dispute.
"The Greater Anglia plans are all part and parcel of a national drive to de-staff our trains and stations as the private train companies look to smuggle in a faceless railway in a dash to fatten up their own profit margins."
The rail company has until June 19 to confirm that no jobs will be lost
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A Greater Anglia spokesman said: "We have no plans to reduce the number of Greater Anglia ticket offices to seven. However, we are currently reviewing responses to a public consultation about proposals to close seven little-used ticket offices.
"There are no current proposals to close any other ticket offices.
"We will continue to consult with our colleagues and the trades unions on any proposed changes to ticket office arrangements."