A US Secret Service laboratory director was charged with committing perjury during his testimony at the trial of lifestyle trendsetter Martha Stewart and her broker, federal prosecutors said on Friday.

Mr Larry Stewart, who testified as an expert witness about the ink on a broker's worksheet, was charged with two counts of perjury in a complaint filed in Manhattan federal court.

Mr Stewart, 46, of Bethesda, Maryland, surrendered on Friday to the Secret Service, according to court documents.

He is not related to Martha Stewart.

Stock in Martha Stewart's company rose after reports of the perjury charges.

Prosecutors later said the convictions of Stewart and Bacanovic should stand, although some legal experts had their doubts.

The perjury charges were related to testimony February 19 and February 25, in which Mr Larry Stewart said he was involved in the original examination of the worksheet.

He was actually involved in a later examination of the paperwork, the documents said.

Manhattan US attorney Mr David Kelley said the alleged false testimony should have no impact on the Martha Stewart and Bacanovic convictions.

"The discovery that Mr. Stewart testified falsely in no way comprises the guilty verdicts returned against either defendant," he said in a letter sent to judge Miriam Goldman Cedarbaum.

But Mr David Howard, a partner at international law firm Dechert who specializes in white-collar criminal defence and corporate investigations, was not so sure.

"She may well be entitled to a new trial if the judge found that some perjured testimony could have contributed to the guilty verdict against her," Howard said.