The fall in journeys is mostly to the City and central London
The London Underground has recorded its biggest monthly fall in journeys for 20 years.
Commuters made 190,000 fewer journeys a day in August. Transport for London blamed the recession and job cuts.
Journeys to central London and the capital's financial district were particularly affected.
August figures were 6.4% lower than the same month last year. The last time such a drop occurred was in the late '80s, Transport for London (TfL) said.
The loss in revenue and gap in budget will be factors for Mayor Boris Johnson to consider when he decides next year's fares, TfL said.
The company said until the last few months, up to 3.5 million journeys were made on the Tube each day. It said figures had been falling for several months, but the drop in August was the largest for some time.
The fall in journeys comes in contrast to 2008's figures. In that year, LU carried about 1.09 billion passengers - the highest in the Tube's 146-year history.
A TfL spokesperson said: "The last time we saw a 6% (drop) in year on year Tube numbers would have been the late '80s and early '90s."
He added: "Fares are a decision for the mayor. He will make his decision later this year, based on our awareness of the need to balance revenue and ridership figures with the need to sustain vital transport investment and improvements for Londoners.
"Tube passenger numbers are down slightly, due to the recession, in line with lower employment. Clearly, this means reduced revenue from fares and has an impact on TfL's finances, now and in future."
Jo deBank, from passenger watchdog London TravelWatch, said: "We don't want to see anyone priced off from transport in London. We would be disappointed if fares rise above inflation."
In January, the mayor announced fares for the city's public transport which rose by an average of 6%.
And Mr Johnson said it was "no secret that there are very, very serious funding issues both for TfL and for all the public sector at the moment".
There were "some very tough choices that we're going to have to make if we want to keep up the investment that we need to make in vital infrastructure for London", he told BBC London.
But he did not answer a question asking if this meant above-average fare increases for the coming year.
In June, a London Assembly report said the city's transport network faced a potential funding gap of £1.7bn by 2018, which it claimed could lead to increased fares and reduced services. TfL disputed the prediction.