Delays in handing ambulance patients to A&E departments are getting worse, MPs have said.
In a report on the ambulance service, the Public Accounts Committee (PAC) said too many patients "are waiting too long to be transferred from an ambulance to hospital care, and this situation has got worse" in the last five years.
Transferring patients from an ambulance to A&E should take no longer than 15 minutes but just 58% of transfers were completed within this time in 2015/16, it said.
This compares with 80% in 2010/11.
Such delays affect patient care and stop paramedics getting back on the road to attend to other patients, MPs said.
"NHS England told us that ambulances not being able to offload patients is one of the most serious concerns in the urgent and emergency care system currently, and to address this issue much firmer performance management of the system is happening," they said.
"After the transfer is complete, ambulance crews are expected to make their vehicle ready for the next call within another 15 minutes.
"Ambulance crews are failing to achieve their own 15-minute standard, adding to the delay. In 2015/16, this was achieved in just 65% of cases."
In 2011 MPs recommended the development of an indicator to measure transfers, but this has not happened.
Wednesday's report also pointed to significant variations in ambulance response times to emergency calls.
Looking at staffing, it said ambulance services "have struggled to recruit and retain staff, and staff shortages are exacerbated by many trusts having high sickness absence rates" - up to 6.7% in some trusts.
The report concluded: "Since this committee last examined ambulance services in 2011, funding increases for the urgent and emergency services provided by ambulance trusts have not kept up with increasing demand; ambulance trusts increasingly struggle to meet response-time targets, despite focusing on these targets to the detriment of wider performance; and significant variations between trusts, in both operational and financial performance, persist or have got worse as insufficient work has been done to understand and reduce variation.
"Action is being taken by NHS England, NHS Improvement and ambulance trusts to address the performance and long-term sustainability of the ambulance services but it has taken too long to begin addressing the issues identified by this committee in 2011."