The trust said it was reaching patients quicker than ever before
Yorkshire Ambulance Service has been given seven months to improve its response times after "consistently" failing to meet national targets.
Last year, the ambulance trust admitted response times were "not good enough" after its service quality was rated as weak by the healthcare regulator.
The Care Quality Commission (CQC) has now told the trust it must meet national targets by 31 October.
The trust said it was 23 seconds short of the targets but was improving.
Yorkshire Ambulance Service (YAS) is one of 10 NHS trusts that have had legal conditions placed on their registration with the CQC under a new, tougher system.
The CQC can give warning notices or fine trusts which fail to meet standards and, in serious cases, can also prosecute them, close them down or restrict their activities.
The commission said YAS had failed to meet government targets requiring 75% of emergency call-outs to be met within eight minutes.
Jo Dent, the CQC's regional director, said the trust had worked hard to make improvements, but it still had "some way to go".
She said "Getting to patients quickly is absolutely fundamental for ambulances. This trust has consistently failed to meet targets.
"This means that patients may not be receiving treatment properly."
YAS had told the CQC it expected to meet the national requirements by next month.
The CQC said it had set a "more realistic" deadline of 31 October.
Martyn Pritchard, YAS's chief executive, said there had been a "significant increase" in the number of emergency incidents in the past two years.
He said: "Year-on-year we are reaching patients more quickly than before.
"Last month the trust reached 75% of patients in Yorkshire with potentially life-threatening illnesses or injuries within eight minutes and 23 seconds.
"We are committed to making further improvements and look forward to demonstrating this to the CQC."