In the mammalian hippocampus, CA1 place cells fire sequentially at behavioural time-scales (hundreds of milliseconds to seconds) when animals explore their environment. The same sequences repeat at a much faster time-scale (tens of milliseconds) co-occurring with sharp-wave ripples during periods of rest. These fast time scale sequences can also occur in the reverse order, but the mechanism behind this intriguing observation remains unknown. Here, we propose that forward to reverse sequence flip can be explained by changing the E/I ratio of the presynaptic CA3 inputs. We first tested the hypothesis in silico. We observed that in the absence of inhibition, inputs of varying synaptic strengths evoked spikes in the order proportional to their strength - largest input evoked the fastest spike and vice versa. On the other hand, the presence of precisely balanced inhibition delayed the spikes in a manner proportional to the synaptic strength - largest input evoked the most delayed spike and vice versa - hence flipping the sequence. We then show this flip in mouse neurons in hippocampal slices using dynamic clamp based inputs. Thus, using both theory and experiments, we show that the long-standing problem of forward-reverse replay sequence flip can be solved using a simple biophysical model of E/I ratio change, with testable predictions for in vivo measurements.