Lewes prison has been labelled the most violent in the country.

The Prison Service's annual report reveals the serious assault rate was worse inside the category-B local jail last year than in such notorious institutions as Brixton, Wandsworth and Wormwood Scrubs.

But Lewes prison chiefs have insisted the figures are unfair and misleading, failing to recognise recent improvements - including last month, when there were no attacks at all.

The prison also fell below national targets for cutting positive drug tests and providing inmates with meaningful work.

Of all open prisons, Ford, near Arundel, was found to be giving prisoners the smallest amount of work to do - but, in contrast to Lewes, suffered no serious assaults at all.

Over-crowding - a rising problem across the prison population - meant Lewes had an average 498 prisoners last year, 13 above its certified normal accommodation.

There were 26 serious assaults recorded, with only Blakenhurst (31) in Redditch and Wormwood Scrubs (32) in west London higher.

But both prisons are significantly larger than Lewes.

In a table measuring serious assaults as a proportion of the average population, Lewes came top with 5.22 per cent.

The national average was 1.54 per cent.

Lewes prisoners carried out an average 21 hours of purposeful activity each week, compared to a national average of 23.2 hours and a target of 24 hours. That gave Lewes the seventh best record among the country's 32 male local prisons.

It was only 22nd best, however, on drugs, with 18.9 per cent of tests proving positive.

Lewes deputy governor David Ward said the figures were not a reliable indicator of a prison's performance.

In quarterly performance ratings, Lewes stood sixth out of 137 prisons and was commended for improvements under governor Eoin Mclennan-Murray, who took over last winter.

Mr Ward said: "Statistics don't tell the whole story. In March we had one prisoner put into a segregation area who then assaulted three members of staff. That counts as three assaults.

"Most attacks are carried out by one or two prisoners but the tables don't reflect blips like that. Other prisons may not be recording assaults as well as we do. In June we had no assaults at all.

"Lewes has a specific drug test target of 15.3 per cent and we're currently running at nine per cent. A lot of progress is being made, as shown by the fact we're sixth in the country."

Ford is under the threat of being put out to private tender if Prison Service director-general Phil Wheatley deems recent progress has not been sufficient.

In his annual report, he revealed Ford provided inmates with the least amount of purposeful activity of the country's eight male open prisons.

New governor Fiona Radford has made this one of her priorities for improvement.

She is also stepping up anti-drug patrols and testing after Ford prisoners failed 23.4 per cent of tests last year.