Recent examples cited by police include a man who saw a pounds 10,000 arson attack - his family refused to give evidence after receiving numerous visits from undertakers and decorators - and a witness to a trailer theft whose home was burnt down the night before he was due to give evidence.
Senior officers want public recognition of the size of the problem of keeping vital witnesses free from interference - said by some sources to be equal in cost to that of protecting politicians - and government support to widen the scope of protection now available.
'People are being threatened out there,' said Det Supt Peter Beardon, deputy head of CID for Avon and Somerset, whose chief constable, David Shattock, recently highlighted the issue in his annual report. 'I think that if the will was there and society accepted the size of the problem, we could do a lot to protect those people who want to give evidence but are afraid they may be subjected to violence or threats.'
The lack of specific legislation and funding for a national protection scheme is at the core of police concern, as the present ad hoc system is under increasing strain from the rise in intimidation. Substantial funds must be set aside across the country, police believe.
All forces can make vital witnesses and informants under threat 'vanish' into new identities. The costs are high - an average of pounds 300,000 a case - but the numbers are few and almost always involve people whose position in or on the edge of the criminal underworld makes them vulnerable. The real demand is for lower- level but time-consuming protection.
'Mostly people just need an officer in their home or at the very least an alarm or a telephone - that is what we need to be able to provide,' said Mr Beardon.
The Independent on Sunday has learnt that Home Office officials are giving 'active consideration' to the problem, possibly by issuing new guidelines to police forces. A new offence of intimidating a witness, juror or anyone assisting the police is already included in the Criminal Justice Bill now passing through Parliament.
Senior officers believe they only hear about a small proportion of intimidation cases, because sometimes key witnesses change their story, refuse to give evidence, or simply fail to turn up in court without explanation.
Mr Beardon said there were 20 to 25 cases in his area last year, possibly about one- fifth of the real total. This suggests that, across the country, the figure could run into thousands, with the collapse of hundreds of trials.
The problem has been aggravated by new rules forcing police to disclose all information obtained during an inquiry.
Last year, the Independent on Sunday revealed that three important Metropolitan Police informants re-settled under new identities abroad had been 'executed'. This occurred after police were forced into admitting that informants had been involved by dropping prosecutions rather than obey court orders to give their details.
In evidence to the House of Commons home affairs committee's investigation into organised crime, chief constables say the provision of a national structure is vital.
'Specific legislation and funding is a crucial part of maintaining the integrity of the criminal justice process. The current ad hoc arrangements have served us well, but are now under strain,' they say.
Only two forces - Greater Manchester and the Met - have formal witness-protection schemes, which can involve relocation.
Senior officers say that the costs of extending the schemes across the country would be high, but must be seen in the light of the outlay on similar protection for politicians.
One officer said: 'We have to ask ourselves whether a system that can offer that to its politicians cannot also do the same for its ordinary victims and witnesses of crime - and make a significant impact on the criminal justice system.'