The trust has improved its financial management
The Yorkshire Ambulance Service NHS Trust has been warned it must improve its performance or face action by a health watchdog.
It comes after the trust was rated "weak" for its quality of services for the third year in the annual ratings by the Care Quality Commission (CQC).
The trust's finance management score improved from "fair" to "good".
Its chief executive Martyn Pritchard said recent improvements had not been reflected in the 2008/2009 results.
Yorkshire Ambulance Service (YAS) is one of three ambulance trusts out of 11 nationally to be rated weak for services.
Mr Pritchard said: "Due to tougher A&E response measurements, introduced last year, the improvements we have made more recently are not reflected in the 2008-09 outcome for quality of services.
"However, we are currently preparing our mid-year declaration to the CQC for 2009-10 and the improvements we have made will be clearly demonstrated."
Sue McMillan, regional director of the CQC in the Yorkshire and Humberside region, said: "Yorkshire Ambulance Trust continue to be rated weak for its quality of service.
"However, there are positive signs of improvement from inspection activity this year and we will be seeking further evidence of improvement by the end of the year."
The CQC said this year it had produced the most comprehensive and rigorous assessment ever of the performance of both mental health and ambulance trusts.
Ms McMillian said the trust, along with others nationally, must address shortfalls in their performance or show processes were in place to improve.
"If they can't do that, it will then be up to us to decide whether to use our increased powers including restricting, suspending or, in the most serious cases, removing their registration."
It is the fourth year the healthcare regulator has carried out its annual assessment of the NHS.
It is based on the trusts' performance between 1 April 2008 and 31 March 2009 and rates each organisation as excellent, good, fair or weak on both its quality of services and financial management.