The number of police forces in England and Wales could be slashed to 20 in just a year's time.

Home Secretary Charles Clarke said the existing 43 forces could be reduced to 20 - the first time he had named a figure since a report signalled the changes last Friday.

Senior police adviser Denis O'Connor later said he wanted to see the radical changes implemented within a year to 18 months.

Mr O'Connor, who wrote last week's report recommending the creation of "strategic forces", said: "I hope it doesn't take more than a year to 18 months from start to finish."

Mr Clarke told the Police Superintendents' Association (PSA) conference in Warwickshire by video link that he wanted to see forces coming up with firm plans as quickly as possible.

He said he wanted to see "probably of the order of 20 to 25 across the country, but I'm not looking at a particular figure".

He said he rejected the PSA's proposal for a single police force for England and Wales.

"A jump straight from where we are to a national police force would be a step too far," he said. "If we were to move down that route the implication is that national politicians could have responsibility or influence for operational matters. That would be a dangerous course."

He also told delegates that he was still not in favour of changing the law to allow telephone tapping evidence to be routinely used in court.

"I'm still not convinced it's the right way to go but we are continuing to examine it to see if that position can change," he said.

Some sections of the intelligence community are known to oppose a law change on so-called " intercept" evidence for fear it will expose their techniques and capabilities.

Mr O'Connor's 113-page report advocated merging smaller police forces, which often struggle to deal with terrorism and serious crime.