An estimated £21.5 million was spent by the Crown Prosecution Service in the past year preparing cases which were never heard in court, new figures reveal.
Around a quarter of this related to cases which collapsed due to "prosecution reasons" - including prosecution witnesses not turning up, and incomplete case files.
The figures were released by the National Audit Office (NAO) as it accused the criminal justice system of failing to deliver value for money, with two-thirds of cases not progressing as planned.
The NAO also criticised a postcode lottery in the system, with a 70 per cent chance of a trial proceeding as scheduled in North Wales, compared to just 20 per cent in Greater Manchester.
Case backlogs in the crown court increased by a third between March 2013, the figures show, while the waiting time from a hearing rose from 99 days to 134 in the same period - a jump of 35 per cent.
"Continuing financial pressure" was blamed for part of the problem, with a real-terms fall of 26 per cent in spending on the justice system.
And while more cases are being dealt with at magistrates court level, the report found, the type of cases being heard are more complex and resource-intensive - such as historical child sex abuse allegations, terrorism, and organised crime.
This led to the average length of a crown court trial increasing from 11.5 hours in 2010/11 to 14.6 hours in the year ending in September.
The NAO concluded that "ambitious" government reforms led by the Ministry of Justice, CPS and judiciary have the potential to improve value for money - but warned they "will not address all of the causes of inefficiency".
A spokeswoman for the CPS said "significant work" had been undertaken to improve efficiency.
A Ministry of Justice spokeswoman added: "We welcome this report and will reflect on its recommendations.
"The MoJ is embarking on a radical reform of the criminal justice system. As the Justice Secretary has said, our criminal justice system is in need of urgent reform.
"Our courts are archaic and slow, and their out of date processes do not meet the needs of the public.
"That is why we are investing a record £700 million to build a justice system that is swifter and more certain.
"We will use modern technology to meet the needs of everyone who uses our services."