Tate director Sir Nicholas Serota has warned that if the gallery can't raise the bulk of the funds it needs for its Transforming Tate Modern project within the next year, "then there's no point in continuing".
In July Tate revealed the revised designs for its ambitious expansion of the former Bankside Power Station building.
So far £70 million – including £50 million from central Government – has been pledged out of a total of £215 million at 2012 prices.
According to Bloomberg, Sir Nicholas told a press conference this week that "it would be very nice to be in a position where we had all the money before we started to lay the brick, but I don't think there's ever any project that is in that position".
Sir Nicholas told journalists that the next 12 months will be crucial in gathering funds for the scheme, and that if only half the cash has been raised in a year's time with no prospect of the balance being achieved, "then there's no point in continuing".
Serota and his colleagues remain confident that despite the challenging economic climate they stand a good chance of raising the cash and conversations are already under way with a number of prospective benefactors.
Tate's annual report reveals that 5.2 million people visited the modern art gallery on Bankside between April 2007 and March 2008.
Tate also says that it is still investigating fundraising options for its proposed National Art Collections Centre in Mandela Way, a joint initiative with the National Portrait Gallery. The scheme, designed by Grimshaw Architects, was turned down for funding by the Heritage Lottery Fund.
Story updated Monday 8 September.