Gibson claimed Ian Elliott 'understated' the details of a proposed new contract to England player Stewart Downing. He told a jury how he was 'uncomfortable' working with Elliott as negotiations over a new contract for the midfielder stalled.
Elliott, 53, is on trial at York Crown Court accused of siphoning off large amounts of money from a bank account held by Downing.
Boro boy: Middlesbrough chairman Steve Gibson has addressed the court as part of the case between Stewart Downing and his agent
Gibson told the court how Downing was one of the best players at the club in 2007 and they were keen to offer him a new, improved contract.
He told the jury how he became increasingly frustrated at the lack of progress on a new deal and decided to deal with Elliott himself.
However, he said he was not prepared to pay Elliott the usual fee of four per cent to five per cent the contract, claiming the agent was 'obstructive'.
He said: 'The relationship with Elliott was such I was not going to pay that, for several abusive phone calls and him being obstructive. I slimmed down the offer to ?50,000 per annum.'
Gibson said Downing became the subject of transfer speculation to Tottenham, confirmed by Elliott on a local radio show.
Gibson said it was 'rule number three in the agent's book' and a way of increasing pressure on Middlesbrough. He said he had an 'unpleasant' telephone conversation with Elliott following the radio broadcast.
Disagreement: Ian Elliott (left) is accused of falsely siphoning off ?500,000 from footballer Stewart Downing
The jury was told Downing was happy at Middlesbrough and 'no one believed he wanted to leave'.
Gibson accused Elliott of having 'no respect for the consequences of his actions towards the club and no respect for the consequences of his actions towards the player'.
'It was all about Mr Elliott and what he could profit from,' he added. He said following a 'very unpleasant' telephone conversation with Elliott he decided to deal with Downing directly.
'I decided at the end of the conversation there was no point talking to him, our best interests was speaking to Stewart and his family direct.'
During cross-examination Robin Patton, for Elliott, put it to Gibson that in a newspaper interview in 2006 he described Elliott as 'one of four agents he was prepared to deal with'.
Star turn: Downing was one of the brightest talents to emerge from Boro's excellent academy
'I have every right to change my opinion.' he responded.
Gibson said he met Downing and his father one evening and offered him ?30,000 a week, rising to ?40,000 a week. He eventually signed the ?11million deal in February 2008.
In an often heated exchange with Patton, he added: 'I went to the boardroom of Middlesbrough Football Club and sat down with Stewart Downing and his father. We told them the contract offer. I was expecting a long meeting, the meeting was short because they were astounded at the figure in front of them.
'It was clear from the discussion the information passed to Downing's family had been corrupted by Mr Elliott, understated.'
Earlier, Downing, 26, who now plays for Aston Villa, told the jury how he discovered just ?11,000 in one of his business accounts.
He said the account should have had ?600,000 to ?700,000 in it.
Elliott, of Foxhills Covert, Newcastle, denies fraud.
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Explore more:People: Stewart Downing, Ian Elliott Places: Newcastle, United Kingdom Organisations: Middlesbrough Football Club