Online chats between colleagues said to contain boasts such as 'our six-month fixing moved the entire fixing hahahah'

A former trader at Royal Bank of Scotland sent mocking emails as he attempted to manipulate the price of Libor, according to court filings in Singapore which add to the controversy surrounding the benchmark interest rate.

Inspected by the Bloomberg news agency before they were sealed, the filings also show an RBS trader quipping "hahahah", describing Libor as a "cartel" and claiming that hedge funds would be "kissing" a colleague if the rate was reduced.

The remarks are contained in instant messages, similar to emails, some of which were sent in the months before RBS was bailed out with £45bn of taxpayer funds in October 2008.

With reforms to Libor expected to be unveiled on Friday, the filings in the Singapore court show how traders around the globe appeared to find it easy to move the interest rate benchmark for at least four years.

The filings are part of a sworn statement by Tan Chi Min, senior trader at RBS in Singapore until he was fired last year for attempting to manipulate Libor. The 231-page statement is part of his case for wrongful dismissal, claiming the bank condoned manipulation of the rate and sought out scapegoats.

Bloomberg reported that RBS, in a letter dated 29 August 2011, sent Tan copies of instant message chats he had with others as evidence of potential wrongdoing and informing him the bank was bringing disciplinary proceedings against him.

In April 2008, Tan sent an instant message to a number of traders, saying: "Nice Libor ... Our-six month fixing moved the entire fixing hahahah."

In an earlier message to colleagues and traders at other banks, including Deutsche Bank, Tan writes: "It's just amazing how Libor fixing can make you that much money or lose if opposite ... It's a cartel now in London."

"What's the call on Libor," one Singapore trader asked a London-based trader in a August 2007 chat.

"Where would you like it, Libor that is," the London end replied.

"Mixed feelings, but mostly I'd like it all lower so the world starts to make a little sense," another trader responded.

"The whole HF world will be kissing you instead of calling me if Libor move lower," Tan said, referring to hedge funds.

"OK, I will move the curve down 1 basis point, maybe more if I can," the London man replied.

RBS, 81% owned by the taxpayer, expects to be fined as a result of investigations by a number of regulators. The scale of the penalty is not yet known. Barclays is the highest-profile bank to be penalised so far, with a £290m fine after regulators in the UK and the US found its traders offered each other bottles of Bollinger champagne for attempting to move the rate. The case also forced chief executive Bob Diamond and other top bankers out of Barclays.

While the identities of individuals involved in the Barclays case were concealed, the documents in Singapore offer no such protection, citing messages with a number of former RBS staff and traders at other banks.

RBS has asked for the documents to be sealed by the Singapore high court while investigations by the FSA and US regulators such as the Commodity Futures Trading Commission and the Department of Justice are completed. But Bloomberg said it had inspected the filings before they were closed to viewing.

"Our investigations into submissions, communications and procedures relating to the setting of Libor and other interest rates are ongoing. RBS and its employees continue to cooperate fully with regulators," RBS said.

The process of setting Libor, which determines borrowing costs on $300tn of financial products around the globe, is expected to be overhauled following recommendations to be set out on Friday by Martin Wheatley, top regulator at City watchdog the FSA. The British Bankers' Association is to lose its role in setting the rate to a formal body. It is also expected the rate will be set on levels at which banks have borrowed rather than their predictions. At present a panel of banks is asked the pricethey expect to borrow over 15 periods, from overnight to 12 months, in 10 currencies.

The instant messages released in Singapore quote Tan as saying in May 2011, when investigations into Libor had begun: "This whole process would make banks pull out of Libor fixing.

"Question is what is illegal? If making money if bank fix it to suits its own books are illegal ... then no point fixing it right? Cuz there will be days when we will def make money fixing it."