Members of an organised criminal gang at the heart of a £57 million conspiracy to "plunder" British museums of rhino horn and other priceless Chinese artefacts have been jailed for up to six years and eight months.

The group, dubbed the Rathkeale Rovers because of their links to the Irish town, targeted high-value objects in a string of break-ins, including Cambridge's Fitzwilliam Museum and Durham's Oriental Museum in 2012.

Judge Murray Creed heard that although the items stolen in Durham and Cambridge were valued up to £18 million, detectives believe they might have fetched more than three times that figure on the booming Chinese auction market.

Members of the same gang also masterminded a bungled attempted theft at Gorringes Auction House in Lewes, East Sussex, and organised the disposal of stolen artefacts in what the judge said was "an extremely sophisticated conspiracy".

Sentencing seven of the 14-strong gang, Judge Creed said: "It is a conspiracy both sophisticated, skilled and persistent, and involved significant cultural loss to the UK of museum quality artefacts and items from international collections."

In all, 13 men are being sentenced over two days, after three trials which concluded with the gang and its associates convicted of wide-ranging criminal conspiracy to steal, with connections to Ireland, Europe and China.

The judge began by jailing Richard "Kerry" O'Brien Jr, 31, of Cambridgeshire - also of Rathkeale in the Irish Republic, for five-and-a-half years.

His uncle, John "Cash" O'Brien, aged 68, of Fifth Avenue in Wolverhampton in the West Midlands was jailed for five years and three months.

Also in the dock was Daniel "Turkey" O'Brien, 45, and Daniel Flynn, also 45, both of Orchard Drive, Smithy Fen, Cottenham, Cambridgeshire, who were jailed for six years and eight months and four years, respectively.

The judge said he had found Flynn to have played "a leading role", but reduced the man's sentence based on "the fragility of his mental health".

Alongside the men in the dock was 56-year-old Donald Wong, of Clapham Common South Side in Lambeth, London, described by the judge as "a buyer, seller and valuer". He was jailed for five-and-a-half years.

Paul Pammen, aged 49, of Alton Gardens in Southend-on-Sea and 37-year-old Alan Clarke, of Melbourne Road in Newham, London, who was said to have headed the gang's "disposal team", were also both jailed for five-and-a- half years each.

Six other men convicted over the conspiracy will be sentenced on Tuesday, including 47-year-old Richard Sheridan, of Water Lane, Cottenham.

Sheridan is a former spokesman for the Dale Farm travellers site in Essex and was seen in Wong's company, shortly before £50,000 in cash was found in the Chinese businessman's car.

The judge said the operation to "plunder" rhino horn, carved horn and jade items started off "small-scale" in January 2012, but that after initial failures and botched thefts - in one case the burglars forgot where they had hidden their haul - "planning paid off".

"It was serious organised crime," he added.

In their most successful theft 18 pieces of Chinese jade were stolen from the Fitzwilliam Museum and although experts provided various valuations up to almost £18 million the judge described them as "priceless".

He added: "They were part of a national collection split between the museum in Cambridge and the British Museum in London."

Afterwards, that haul was stored in a safe house before being taken by taxi to Purfleet in Essex where the goods were spirited away.

The judge continued: "The conspiracy spanned England, Scotland, Northern Ireland and the Irish Republic, references were made to France - the Cherbourg visit, Hong Kong and also the United States and Germany, also featured in the evidence the court heard over the three trials."

He said the gang had either stolen or tried to steal "highly prized museum-quality" items, often with historic Imperial Chinese dynastic connections, with the exception of an attempted theft on an auction house in March 2012 in which the bungling thieves took the wrong item.

On two occasions the Oriental Museum in Durham was targeted, but also the Castle Museum in Norwich, Gorringes Auction House in Lewes, East Sussex, and the Fitzwilliam Museum.

The men carried out reconnaissance of these and other sites, including three museums in Glasgow, and another auction house in Yorkshire.

Mr Creed said there had been "no expression of regret or remorse" from the men, and there was "no prospect of recovery" for the stolen items.

Due to be sentenced on Tuesday are; Patrick Clarke, aged 34, of Melbourne Road, Newham, London, 35-year-old Ashley Dad, of Crowther Road, in Wolverhampton, and Terrence McNamara, aged 46, of Marquis Street in Belfast.

Michael Hegarty, aged 43, and 26-year-old John "Kerry" O'Brien Jr, both of Orchard Drive, Smithy Fen, Cottenham - but also of Rathkeale, will also be sentenced.

The judge said the conspiracy had been centred on the family seat in Rathkeale, telling the defendants: "At the heart of this enterprise was a family - a number of you are members of the O'Brien family."

He added: "Of 14 original conspirators, seven were connected with that home, seven were associates, like Terence McNamara, while others were recruited in to find thieves prepared to carry out burglaries, particularly in carrying out the second attack on Durham museum.

"No doubt others were involved too."

Robert Gilbert Smith, formerly of Hockenden Lane, Swanley, in Kent, had already admitted his part in the conspiracy and was jailed for his part in the crime last year.