(Reuters) - The British trader fighting extradition to the United States on charges of having contributed to the 2010 "flash crash" on Wall Street told a London court on Wednesday he had done nothing wrong and was just good at his job.

Navinder Singh Sarao, 36, who traded from his parents' modest home in west London, has been charged by the U.S. Justice Department with wire fraud, commodities fraud and market manipulation.

"I've not done anything wrong apart from being good at my job. How is this allowed to go on, man?" Sarao said at Westminster Magistrates' Court.

Two weeks ago, Sarao was granted bail pending a full extradition hearing later this year provided he produced just over 5 million pounds and met other conditions.

But his lawyer James Lewis said Sarao had not been able to access the 5 million pounds because U.S. authorities had frozen his assets.

"We cannot obtain any money. Any request to obtain the money was refused," Lewis told the court.

He asked for the bail surety to be lowered to 50,000 pounds on Wednesday but the application was rejected by District Judge Elizabeth Roscoe, and Sarao remains in custody.

"I will not vary bail. I will leave it as it is," Roscoe told Sarao and his legal team, who said they would consider an appeal to London's High Court.

Sarao, dressed in a grey sweatshirt and grey tracksuit bottoms, had sat calmly in the dock until he heard his bail conditions would not be changed, and then spoke out to the court to plead his innocence.