East Midlands Ambulance Service (EMAS) is expected to fall short of national targets for response times.
The target of getting to 75% of the highest priority emergencies within eight minutes is now "unachievable" by April, according to NHS bosses.
The ambulance trust said if the target was missed, it would be by a "very small margin".
Officials said response times had been affected by snow and ice during recent bad winter weather.
We have been affected by the worst winter weather conditions for 30 years
East Midlands Ambulance Service
The service is now being monitored on a weekly basis.
NHS East Midlands said response times to the two highest priority call-out categories continued to be below national standards.
Figures showed the service had so far achieved 73.5%, compared with the 75% target.
The service was also at significant risk of missing the target of responding to 95% of slightly less urgent call-outs within 19 minutes.
An NHS East Midlands spokesman said: "The key area of Strategic Health Authority focus is now on ensuring EMAS achieves the operational standards on a weekly basis for the remaining weeks of the year."
New ambulances
The ambulance trust said it had a track record of hitting response targets, and the spokesman added: "We have the 75% target in our sights and if we miss it, it will be by a very small margin.
"We have been affected by the worst winter weather conditions for 30 years.
"During the worst periods of snow and ice, we said we would get to everyone who needed help.
"Our staff showed great commitment and we are sure the public will appreciate they did a tremendous job in very difficult circumstances."
The service has also unveiled a new £9m fleet of 91 ambulances, which staff have helped to design.
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