Harry Redknapp told police during interviews he "writes like a two-year-old", a court has heard.
Tottenham boss Mr Redknapp and Milan Mandaric both deny charges of cheating the public revenue during their time together at Portsmouth.
But Mr Redknapp told officers: "I am not going to fiddle taxes, I pay my accountant a fortune to look after me.
"I am completely and utterly disorganised. I write like a two-year-old and I can't spell."
Jurors have also heard the football boss had made "disastrous" business decisions, including losing £250,000 in a "very unsuccessful" takeover bid at Oxford United.
Earlier the court heard Mr Redknapp had sole responsibility for a Monaco bank account in the name of his dog.
The prosecution alleges Mr Mandaric made untaxed payments totalling $295,000 (about £189,000) into the Monaco account when he was chairman and Mr Redknapp was manager of the south coast side.
In the recorded interviews played to the court, Mr Redknapp said: "I have never written a letter in my life."
He said his accountant "runs my life" and was so disorganised he had not realised that he had not been paid for 18 months for his column in The Sun newspaper, it was heard.
He also told officers of a feud with Mr Mandaric over a bonus he was due for the sale of Peter Crouch from Portsmouth to Aston Villa.
Mr Redknapp added: "I was getting bigger than him [Mr Mandaric] at the football club and he didn't like it really."
He said his co-defendant had not wanted him to sign Crouch in the first place.
"Mandaric told me 'He's useless. I think you are going to have to pay me 10%'," Mr Redknapp told police.
When England striker Crouch was sold on for a £3m profit, Mr Redknapp said he was due 10%.
The court has already heard he was unhappy after his cut of transfer profits was reduced to 5% when he took on the managerial role at the club.
But Mr Mandaric paid $145,000 (£93,100) into the Monaco account as an extra "bonus" for the deal, according to Mr Redknapp, although Mr Mandaric claimed this was an investment, jurors have heard.
The football boss said he then told Mr Mandaric: "I don't want to end up with a tax bill."
The defendant said he had been told by Mr Mandaric there was no tax "on so many occasions".
During the interviews, Mr Redknapp also spoke about leaving Portsmouth in 2004.
He said the team were 2-0 down to Aston Villa at half-time and Mr Mandaric tore his ticket "into 50 pieces".
The next day Mr Redknapp wanted to leave the club, the court heard.
He was offered about £200,000 as a settlement but asked for the money to go to youth football projects in Portsmouth, the jury heard.
Earlier, the court was told Mr Redknapp's signature was the only one on records for the Monaco bank account named "Rosie 47" after his dog and year of birth.
Mr Redknapp, 64, who lives in Poole, Dorset, and Mr Mandaric, 73, of Oadby, Leicestershire, each deny two charges of cheating the public revenue.
The first of the two charges alleges that, between 1 April 2002 and 28 November 2007, Mr Mandaric paid $145,000 (£93,100) into a bank account held by Mr Redknapp in Monaco, to avoid paying income tax and National Insurance.
The second charge for the same offence relates to a sum of $150,000 (£96,300) allegedly paid by Mr Mandaric to the same account between 1 May 2004 and 28 November 2007.
The trial continues.