Edinburgh Zoo has warned that the chances of Britain's only pandas successfully mating this year "have decreased."
Yang Guang and Tian Tian may be given one last opportunity tomorrow morning.
Female pandas only ovulate once a year.
Britain's only pandas have just one more chance to breed successfully, as time runs out.
On Wednesday "natural sparks" flew between Yang Guang and Tian Tian, but the pair failed to mate, as the female bear's limited window of fertility draws to a close.
Keepers at Edinburgh Zoo opened a "love tunnel" between their enclosures after tests showed Tian Tian had ovulated.
They met five times on Tuesday and "several times" on Wednesday but failed to ignite the "spark".
The pandas will have a final chance to mate on Thursday but keepers are losing hope that Tian Tian will get pregnant this year.
Breeding pandas in zoos is crucial to saving them from extinction. Actually, no not really.
The clock is ticking for Yang Guang and Tian Tian to mate within a limited window of fertility.
The last time Julie Etchingham reported on pandas was when she worked on BBC's Newsround in the mid 1990s.