Only two spaces will be provided in the trains' carriages between Edinburgh/Glasgow and Aberdeen/Inverness compared to four at present.
A further six spaces will be available in separate compartments at each end of the trains, but only for cyclists making the full journey.
The plans follow a proposed reduction in cycle spaces on the Glasgow-Oban line, which ScotRail is re-considering.
Lothian cycle campaigners Spokes has launched a campaign against the proposed cuts to the refurbished 40-year-old InterCity 125 High Speed Trains (HST).
It said extra spaces could be created by converting some of the toilets, that are due to be left locked and unused.
It said: "The bike space promise has gradually been whittled down - and now we discover that for many journeys there will be fewer spaces even than at present.
"Meanwhile some redundant toilets will be locked away, just transporting air - surely an opportunity to greatly increase bike spaces for trips between intermediate stations.
"Bike/rail capacity to the Highlands is a major concern, not just for commuting and leisure cyclists, but also for the tourist industry.
"Surely the power car door could be unlocked when necessary, at least at main stations such as Perth."
Spokes said ScotRail had told MSPs in 2015 to expect “at least 20 cycles” per train in the new fleet.
However, it said there would now be space for just two bikes on a vertical-hanging rack, converted from a toilet in one of the coaches.
There will be space for three more in each of the luggage compartments beside the power cars at either end of the four and five-carriage trains, for end-to-end journeys, such as Edinburgh to Inverness, but these would not be accessible at intermediate stations.
Scottish Labour transport spokesman Neil Bibby, who has taken a close interest in ScotRail's performance, said: "At a time when we should be promoting cycling, reducing bike spaces on these new trains would be a retrograde step.
"The public were promised action on overcrowding and more space for cyclists.
"The promises that have been made to passengers should be kept."
ScotRail said its plans for the six spaces in the power car storage area were under review.
A spokeswoman said: "We have to balance a number of demands when planning space in our trains and want to accommodate bikes as far as possible.
"We’re exploring a number of options in advance of the HST fleet arriving in Scotland.”