Almost half of Britain’s police forces lack the resources to deal with serious threats to the public such as terrorism or organised crime, a report by the official watchdog has warned.

It follows the failure of plans to merge forces such as West Midlands Police with smaller neighbours, including West Mercia, Staffordshire and Warwickshire.

The Home Office announced in 2005 that it wanted smaller forces to merge, after an inquiry warned they lacked the resources and specialised knowledge needed to combat major threats to public safety.

But the proposal was scrapped a year later.

It followed opposition from forces including West Mercia, as well as fears that the reforms would lead to higher council taxes.

Instead, Ministers urged forces to co-operate voluntarily so that they pooled resources and expertise.

The new report by Her Majesty’s Inspectorate of Constabulary warns that half of Britain’s forces have no plans for dealing with serious incidents.

It does not name the forces it is concerned about, although the inquiry was prompted by worries about the performance of smaller forces.

In the West Midlands, these include Staffordshire, with 2,193 officers, West Mercia (2,433) and Warwickshire, with just 999 officers.

West Midlands Police, whichg is one of the largest in the country, employs 8,373 officers.

The report warned: “The decision not to reshape the policing structure ended the mergers debate for the moment. It did not, though, close the gaps in protective services.”

Inspectors examined whether forces were prepared for ten major problems, including organised crime; “critical incidents” such as terrorism; public disorder; “major crime” and missing children. They concluded: “Just under half – 21, or 49 per cent – had credible plans in all of the ten areas. This means, of course, that nearly half did not, a troubling finding.”

Ministers had asked forces to collaborate but some had failed to do this and “looked almost exclusively inwards”, the inspectors warned.

The findings could spark fresh debate, but Ministers will be reluctant to re-open an issue over which they were forced to carry out a major u-turn.

Former Home Secretary Charles Clarke had been adamant that forces would merge if they wanted to or not, but the policy was swiftly changed in 2006. Instead, Ministers now plan to give the Government additional powers allowing the Home Office to order forces to work together.