'Someone within Cosco seems to have drawn a line in its chartering book to decide which charters should be paid and which shouldn't,' said one shipowner.

The shipowner would not speculate on the level, but hire rates for larger vessels have dropped sharply since the downturn in the shipping markets in late 2008.

At the time, the average rate for a 180,000-deadweight-tonne Capesize vessel chartered for three years was US$82,260 per day, but is now US$15,750 per day. Similar charters for a 76,000 dwt Panamax ship has fallen to US$13,375 per day this month from a daily average of US$44,356 in 2008.

The shipowner added: 'Whether that 'someone' was Captain Wei Jiafu [Cosco chairman and chief executive] or executives in Cosco's dry bulk operations will probably never be known.'

Cosco's actions also annoyed Jeremy Penn, the chief executive of London's Baltic Exchange, whose motto is 'Our word our bond', and which monitors chartering activity and the shipping markets.

'The Baltic takes the view that a deal agreed between two parties stands and it's simply not acceptable for one party alone to attempt to change the terms,' Penn said.

Australia's Fortescue Metals Group was suspended as a Baltic Exchange member after it annulled high-rate charter contracts with shipping companies when the dry bulk cargo market collapsed in late 2008. The move led to several shipping companies filing for bankruptcy or protection from creditors.

Other shipowners said Cosco companies had withheld charter hire payments or made only partial instalments to comply with mainland regulations that ended the tax exemption for foreign owners leasing ships to mainland owners.

Marcus Dodds and Li Lianjun, from law firm Reed Smith, said the regulations covering enterprise income tax and business tax required mainland entities such as Cosco to withhold tax from each hire payment. This could amount to 15 per cent of the hire paid.

However, one Hong Kong shipowner questioned whether Cosco's withholding of payment was 'truly legit, and if so, what's the tax rate'.

One banker also wanted to know if Cosco would have reneged on charters 'if it was a Chinese owner backed by Chinese financing?'

Sinotrans Shipping executive director and general manager Tian Zhongshan said the company had 'a few vessels' on charter to Cosco companies.

The firm is an offshoot of Sinotrans & CSC, one of the mainland's largest shipping and logistics groups.

Tian said that while 'pricing was particularly good, [Cosco] was normal and regular in its payments' for charters.

Zhang Liang, the president of Cosco's Hong Kong-listed subsidiary, China Cosco Holdings, said the parent company and its subsidiaries had charters for more than 400 ships but 'the percentage of disputes was small compared with the overall number of charters'.

China Cosco had charters on 201 vessels in the first half of this year, including 95 Capesize and 165 Panamax ships carrying iron ore and coal. This was up from 61 Capesize and 86 Panamax vessels in the first quarter.

Zhang said that settlements had been reached on 18 charters but he gave no details. China Cosco said it had liabilities of about 6 million yuan (HK$7.32 million) from outstanding litigation or arbitration.

One shipowner said a typical negotiating ploy would involve Cosco and a shipowner agreeing on a lower rate on the existing charters and then Cosco agreeing to charter one or two more vessels or extending the original leases by one to three years.

China Cosco made provisions totalling 1.5 billion yuan for onerous contracts in the first six months of this year, compared with 374.5 million yuan a year earlier. The full amount of cash in dispute is unknown because they include charters signed between Cosco's dry bulk cargo subsidiaries that are not part of China Cosco.

Analysts said China Cosco's dry bulk woes appeared to have deepened in the second quarter. They said the company, which included container line and terminal operations, made a net loss of 503 million yuan in the first quarter, rising to 2.24 billion yuan the next quarter.

Several banks and law firms specialising in ship finance thought a Cosco charter used by an owner as collateral for a new ship order was still a bankable proposition.

'I would say that these sorts of actions are just part of the way the shipping world works and just one of the negotiating tactics that players use,' said Davide Barzilai, a partner in law firm Norton Rose. 'Banks that understand the market will no doubt see through these sorts of negotiations.'

His views were echoed by Hong Kong-based shipping analyst Thomas Bodereau, a senior executive at Natixis, the corporate and investment management arm of Groupe BPCE, the second-largest bank in France. He said the firm was not linked to any financing connected to the charter disputes.

'However, Cosco remains for us a good credit,' he said. Asked if Natixis would support a shipowner ordering a Capesize ship with a five-year charter to Cosco, Bodereau added: 'I would say maybe not right now.'