Victims of crime are being let down in areas covered by nearly half of the police forces in England and Wales by poor investigations, a policing watchdog has said.
In its annual assessment of the state of policing in England and Wales, Her Majesty's Inspectorate of Constabulary (HMIC) said it was of "material concern" that 18 forces out of 43 were assessed as requiring improvement in the way they investigate offending.
Inspectors said a "deficit in skill and experience of officers investigating crimes" and a lack of supervision was behind weak investigations.
In the 12 months to March, only one force - Dyfed-Powys - in England and Wales was able to achieve detections, which include cautions as well as charges, for more than half of the crimes recorded. Detection rates range from 22% to 51%.
"Opportunities to secure a successful outcome for victims of crime are being missed as a result of failures to conduct an effective, prompt and professional investigation," the HMIC report said.
Failures to do house-to-house inquiries, take photos of injuries in domestic abuse cases and collect CCTV evidence on assaults in public places were among some of the inconsistencies found in the way evidence was gathered, the report said.
Crimes are being investigated by officers who also provide neighbourhood policing services such as patrols, some of whom have not investigated crimes for a number of years, inspectors said.
Inspection plans - used by officers to determine what they need to do gather evidence - were of a poor standard in 18 forces, with some officers copying and pasting previous versions to new investigations.
HMIC has also launched a new website to allow the public to see at a glance how well their force is performing.
Elsewhere, the inspectorate raised concerns about forces' ability to keep up with modern criminal activity, including cyber-crime and child sexual exploitation.
Chief Inspector of Constabulary Tom Winsor said officers were " policing the crimes of today with the methods of yesterday and insufficiently prepared for the crimes of the future".
HMIC assessed the " effectiveness , efficiency and legitimacy" of each force.
In relation to effectiveness, 40 forces were rated good or outstanding at preventing and reducing crime with three forces - Gwent, Bedfordshire and Humberside - requiring improvement.
Some 41 forces are good or outstanding at tackling anti-social behaviour with two forces - Gwent and Cleveland - requiring improvement.
However, only 24 forces were assessed as good at investigating crime.
In relation to efficiency, 40 forces were rated good or outstanding for the value for money provided with three forces - Bedfordshire, Gwent and Nottinghamshire - rated as requiring improvement.
Shadow policing minister Jack Dromey said: "This report is very worrying - it is just not good enough for half of police forces to be rated less than 'good' at investigating crime.
"This is surely a central plank of policing and raises serious questions for the Government about their approach to cutting crime. Theresa May cannot expect to cut 16,000 police officers and still deliver the highest standards of policing the public want and deserve.
"HMIC is right to draw attention to the changing nature of crime. Labour has been calling for the Government to take action against the rising wave of cybercrime - from fraud to the grooming of children.
"The Home Secretary has ignored this - and, as this report highlights, the police do not have the resource or expertise to respond to the scale of these offences."
Sir Hugh Orde, president of the Association of Chief Police Officers (Acpo), said: "Chiefs whose forces have fallen short on the standard of crime investigation will be analysing the reasons for this and taking action to improve on this crucial area of policing."
Steve White, chair of the Police Federation of England and Wales, said: "We have been warning there are issues around what the service can provide against cuts to funding and police numbers. Although we are pleased that the majority of forces are performing well, we know that officers are struggling to provide the service they believe the public deserve."
:: The 18 force s that require improvement in investigating practices are: