Transport for London has been accused of "making life easy for fare dodgers" by Labour's London Assembly transport spokesperson Val Shawcross.
Using TfL's own data, Shawcross estimates that fare evasion costs the network £61m each year because of ticket barriers being left open.
The data compiled from TfL's regular mystery shopper exercises shows that barriers were left open 6.9 per cent of the time in June and July this year, although some stations had a much higher rate of leaving barriers open for long periods of time.
Bromley-by-Bow was the worst offender, where ticket barriers were left open 61 per cent of the time. In total 20 stations had their gates left open more than a quarter of the time.
The below chart shows the worst offenders across the Tube network. The frequency is in a given quarter across 2015, and only the highest single occurrence is shown - for example, Bromley-by-Bow's worst period was the third quarter, whereas for High Barnet it was the fourth.
TfL rules state that when there are no members of staff available to help passengers, the barriers must be left open.
Shawcross, who has already been a vocal critic of TfL's plans to reduce station staff as part of a network modernisation programme, claimed these plans would exacerbate the problem and lead to "millions more pounds" being lost in unpaid tickets.
She said: "The vast majority of Londoners are honest about these things but there will always be a small minority who take advantage and try to avoid paying their fares. We should be doing all we can to crack down on fare dodgers, not making life easy for them by leaving some station ticket barriers open more than half the time.
“By cutting 950 staff from stations TfL risks having to leave ticket barriers open more often, making fare evasion that little bit easier. It’s a total false economy if the money saved by reducing staff is then lost as a result of increased fare dodging.”
But TfL refuted the complaints, noting that even when barriers were open commuters were required to touch in or out with their Oyster.
Steve Burton, TfL’s director of enforcement and on-street operations, told City A.M: “Fare evasion on our rail and bus networks has fallen to around just two per cent of all journeys, at the same time as overall passenger numbers have risen massively due to London’s growth. This shows that the overwhelming majority of our customers simply want to pay the correct fare and we now have more staff than ever before in ticket halls to assist them.
“We take fare evasion of any kind extremely seriously and communicate the consequences of being caught without a valid ticket. For every journey, customers must have a valid ticket or tap in and out using Oyster or a contactless payment card – even if the ticket barriers are open. If customers do not tap in or out then they will be charged a maximum fare for the journey.”