Ambulances are struggling to hand patients over to A&E departments within target times, MPs warn.
Too many patients are "waiting too long to be transferred from an ambulance to hospital," the Public Accounts Committee (PAC) has found.
Its report on the ambulance service found that the situation has got steadily worse over the past seven 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 was found.
This percentage compares with 80% in 2010/11.
The delays are affecting patient care, MPs say, and stopping paramedics from getting back on the road to other patients.
"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," the report 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.
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.