Staffing levels are to be reduced at about 40 London Underground stations where ticket offices are closing.
Less than 3% of journeys are made on single and return tickets
The success of the Oyster smart card has significantly reduced demand for paper tickets, Transport for London (TfL) said.
From March 2008, about 240 staff will be redeployed from the least busy stops to the busiest stations.
But watchdog London TravelWatch said: "We do not want to see the less busy stations suffer unduly."
Fewer than 3% of Tube journeys are now made on single and return tickets, with Oyster accounting for more than 60% of all trips, said TfL.
It said in addition to the closures to lightly-used ticket offices, other stations will see a reduction in ticket office hours.
"There will be some changes at some of the stations," said a TfL spokesman.
"Some stations such as King's Cross, London Bridge, Bank and Victoria will see an increase in staffing."
The shake-up does not involve redundancies and the redeployed staff will be assigned to frontline services such as driving trains.
"The success of Oyster has led to a huge reduction in the number of customers buying tickets at our stations," said Richard Parry, of London Underground.
"This has meant we have already been able to shift station staff from behind the plate glass windows in ticket offices to the platforms and in ticket halls.
"By increasing the visibility of staff they will make the stations a safer place and will be able to fully assist customers."
TravelWatch said it was "surprised" by the announcement as it was due to meet to discuss the issue with TfL in July.
"We would be concerned if staffing levels fell drastically," said a TravelWatch spokesman.
"There needs to be enough staff for passengers to be and feel safe, secure and for the provision of information."
The Rail, Maritime and Transport (RMT) union branded the move as "unnecessary and dangerous".
"These are straightforward cuts and they affect every Tube worker because they will lead to more ticket disputes, more assaults on staff, more stress, and more of our members working alone," RMT general secretary Bob Crow said.
James Brokenshire, MP for Hornchurch, said "some of the more vulnerable members of our community" would be worst hit by the closures.