NAIVELY, as the muscles rippled and the bat swung and the home runs soared, baseball allowed itself to believe that Barry Bonds was a wonderful freak of nature. Then came the Balco scandal, and now baseball is coming to suspect that natural talent and hard work are not the whole truth in the tale of how the San Francisco Giants star became the sport’s greatest slugger at an age when his faculties should have faltered.

Papers leaked to the San Francisco Chronicle yesterday say that last year Bonds told a federal grand jury investigating Balco that he used a cream and a clear substance during the 2003 season supplied by Greg Anderson, his personal trainer. Anderson is linked to Balco and was charged this year with distributing steroids.

Bonds told the jury he was unaware that he was using steroids and that he only sought to ease his arthritis. “It didn’t even work . . . all I want is the pain relief, you know?” he said. He testified that he was told the substances were a nutritional supplement and a balm for arthritis.

Aged 37, in 2001, Bonds shattered the single-season home run record by hitting 73. The previous year, he hit 49. After 2001, Bonds hit fewer home runs but other facets of his game continued to improve and he cemented his reputation as the most feared hitter in baseball. Last month, aged 40, he won a record seventh National League most valuable player award.

Michael Rains, Bonds’s lawyer, implied that the Government had leaked the documents to smear Bonds. “My view has always been this case has been the US versus Bonds and I think the Government has moved in certain ways in a concerted effort to indict my client,” he said.

The debate about the validity of Bonds’s past and future achievements will thunder on indefinitely. Sportswriters are asking if an asterisk should be placed by Bonds’s name in the record books when, as soon as next year, he eclipses Babe Ruth and Hank Aaron’s totals to become the all-time home run leader. The character would signify: this is a tarnished total.

Bonds is the most significant, but not the only, major league player to be implicated in the Balco affair. On Thursday, papers leaked to the Chronicle revealed that Jason Giambi, the New York Yankees hitter, testified last December that he used steroids and human growth hormone. Giambi said that he obtained steroids from Anderson.

Giambi, who used to play across the San Francisco Bay for the Oakland Athletics, signed a contract with the Yankees in late 2001 that will pay him $120 million (about £62 million) over seven years. The Yankees may now try to void it. Last season the 33-year-old missed half the season suffering from a benign tumour, among other ailments. Gary Sheffield, another elite Yankees player and once a close friend of Bonds, has said that he unwittingly used steroid cream and pills in 2002.

The Balco fallout is forcing baseball to address an issue that it had ignored for decades. The sport introduced only last year a tame steroid-testing policy, which revealed that between 5 and 7 per cent of those tested had taken steroids. “I instituted a very, very tough programme in the minor leagues on steroids in 2001,” Bud Selig, the commissioner of baseball, said on Thursday. “We need to have that programme at the major league level. We’re going to leave no stone unturned until we have that policy in place by spring training 2005.”

An online poll for ESPN found that more than 93 per cent of respondents said that steroid use tainted baseball, but many fans are indifferent, preferring to revel in the prodigious power of the elite players than to mull over the integrity of what they are watching. Even so, for many, the revelations in Bonds’s testimony have poisoned his vast legacy.