An unattended bag on the Tube in London was not checked for several stops - leading a manager to warn of "Russian roulette with people's lives".
An internal e-mail obtained by the RMT union says the suspect bag was allowed to travel four Jubilee line stations before being checked.
Union bosses - engaged in a dispute with Transport for London (TfL) over jobs - claim low staffing was to blame.
But TfL denies this, saying 50 staff were on duty.
The e-mail, purportedly from a Jubilee Line controller, reads: "Canning Town staff eventually made the necessary checks and fortunately on this occasion, it proved to be a customer's bag.
"This is a very serious issue and I think we are beginning to play 'Russian roulette' with people's lives if this sort of failure in a duty of care is allowed to persist."
It continues: "I was on the verge of suspending the railway and calling for the appropriate emergency services to attend.
"In hindsight, I was perhaps remiss in my duty by not doing so.
"In the run-up to the Olympics we cannot afford to become complacent as the threat of a terrorist attack on the Underground remains very real."
RMT general secretary Bob Crow said: "Here we have one of their own senior managers warning that we are playing Russian roulette with people's lives because there aren't enough staff left to carry out basic emergency procedures.
"This memo shows in the starkest possible terms that everything we have warned of in terms of safety and terrorism risks from the Tube cuts programme in the lead up to the Olympics is absolutely true."
'Tried and tested'
But Howard Collins, London Underground's chief operating officer, said: "These claims by the RMT leadership are absurd.
"It is completely wrong to claim that London Underground's security has been put at risk by any staffing changes.
"This is simply a cynical attempt by the RMT leadership to resist the sensible redeployment of staff out of under-used ticket offices.
"We have well tried and tested processes in place to deal with all unattended items and these remain completely unchanged."
There have been a series of strikes over the TfL plan to cut 800 jobs.