Fewer witnesses in criminal cases are turning up at court, a watchdog has warned.
Attendance is falling despite an increase in the number of witness summonses issued, HM Crown Prosecution Service Inspectorate (HMCPSI) said.
Chief inspector Kevin McGinty said: "Fewer and fewer witnesses are attending court.
"Witnesses not attending court can mean that a trial has to be rescheduled, and can even result in the defendant being acquitted."
He added: "The CPS rightly wants to reduce the number of times that trials have to be rescheduled or defendants walking free but there's little evidence that use of the summons is making a difference."
A report by the inspectorate cited figures indicating that a rising proportion of prosecutions are "cracked" - the official term used for a case that concludes unexpectedly - on the day of trial due to a witness being absent or withdrawing their evidence.
It said 2.1% of trials in the crown court and 6.8% of trials in the magistrates court were cracked for this reason in 2014/15, compared with 1.8% and 6.3% respectively in the previous year.
The report also said that more than one in 10 witnesses in domestic abuse cases are being inappropriately summoned to court.
Inspectors thought applying for a witness summons was not the most appropriate course of action in 14 out of 110 cases in a sample.
Courts can issue a summons to require a witness to attend court to give evidence or produce a document or other item. Failure to appear can result in a warrant for the witness's arrest.
The decision to apply for a witness summons "should not be taken lightly", HMCPSI said.
It said: "A witness summons is the last resort for those who disengage from a prosecution and should only be considered when all other avenues have been exhausted. Ultimately it can result in the deprivation of liberty of the victim - often impacting on those who may be the most vulnerable."
A CPS spokeswoman said: "We agree that applying for a summons should be the last resort and that all other avenues must be explored to secure the witness's attendance at court before doing so.
"The final decision on whether a summons is issued rests with the court."
She added: "In 2014/15 a small number of cases listed for trial were ineffective due to prosecution reasons, only 5.2% of total cases listed for trial at the Crown Court and 4.8% at Magistrates' Courts.
"Of these ineffective trials, only a very small proportion were due to non-attendance of victims or witnesses.
"The CPS and police are conducting a joint review of the provision of support to victims and witnesses. This review, which aims to improve levels of service, will be an opportunity to explore the reasons for the drop in witness attendance.
"We will look at ways to ensure that key messaging around witness summonses, and in particular with regard to domestic abuse cases, is reinforced within CPS areas.
"We will be updating our training on domestic abuse over 2016/17 and will consider including training on witness summonses within this."