The ban will be implemented across London's entire transport network
London Underground workers have attacked plans for a public drinks party on Circle Line trains on the eve of an alcohol ban.
The Rail Maritime and Transport (RMT) union said it was irresponsible of those behind the event, when alcohol played a part in trouble on the system.
Saturday's "Last Round on the Underground" party has been advertised on the internet.
The alcohol ban also applies to buses, the Docklands Light Railway and trams.
From 1 June, open containers of alcohol will be banned with the aim of making travelling on London's public transport network a safer and more pleasant experience for passengers.
The new policy was announced earlier this month by London Mayor Boris Johnson.
People have been invited to bring their own cocktails to the party, but to behave themselves and be civil to other passengers.
Bob Crow, general secretary of the RMT, said: "It is pretty irresponsible to organise a booze party on the Tube when alcohol already plays a part in so many incidents.
"We are concerned that London Underground does not intend to put on any extra staff on Saturday night, as it will be our members who will be in the front line and at risk from aggressive drunken behaviour.
"RMT will support any measure that will make our members' working lives safer and more pleasant, but the ban has been poorly thought through, is being implemented in haste and could put our members in greater danger."
A spokeswoman for Transport for London said it would be monitoring the party, but was confident it would pass off without incident.
Addressing the RMT's concerns, she said: "We will not be asking staff to put themselves in a position where they feel they are in danger.
"If they feel they are in danger they can call for extra help from the approximately 2,500 police and community support officers dedicated to the transport network."
British Transport Police said it would have an increased number of officers deployed on the Underground for a number of events in London on 31 May.