Ingrid Lederhaas-Okun, 46, pleaded guilty to theft charges on Friday

'I knew it was illegal to steal, and I did so regardless… I'm very sorry', she told a judge

Former Tiffany & Co vice-president, Ingrid Lederhaas-Okun, pleaded guilty in a New York federal court on Friday to stealing $2.1 million (£1.3 million) worth of jewellery from the fashion house in a case that has shaken the industry.

Appearing in federal court in Manhattan, Ingrid Lederhaas-Okun admitted she had stashed the jewellery in her home in Darien, Connecticut, where she was arrested earlier this month.

"I knew it was illegal to steal, and I did so regardless… I'm very sorry," she told a judge before pleading guilty.

Working in the high-end jewellery house since 1991, Lederhaas-Okun stole a total of 165 pieces, including diamond and gold rings, bracelets, earrings and pendants, in a period of 13 months.

She sold them to an international New York-based company that buys and resells jewellery. Other items were found in her home.

The 46-year-old has been charged with wire fraud and interstate transportation of stolen property.

According to WWD, she faces 46 months in prison following a plea agreement that will forfeit $2.1 million and restore $2.2 million. Her sentencing will take place on 10 December.

The theft was discovered when Tiffany undertook an in-depth inventory in November 2012. Lederhaas-Okun left the company by February this year and on 2 July was arrested.

The items included numerous diamond bracelets in 18-carat gold, diamond drop and hoop earrings in platinum or 18-carat gold, diamond rings in platinum, rings with precious stones in 18-carat gold, and platinum and diamond pendants.

"Diamonds are forever but stolen diamonds are not," said Manhattan US attorney Preet Bharara in a statement on Friday.

"Over a period of years, Ingrid Lederhaas-Okun, an executive at a high-end jewellery company, looted her employer's jewellery inventory and then resold millions of dollars' worth of the merchandise in order to enrich herself. Today, she stands convicted for her thievery and faces the prospect of prison."

Authorities alleged Lederhaas-Okun purposely checked out items valued at under $10,000 apiece to avoid detection.

The company takes a daily inventory of all checked-out items worth more than $25,000.

She will be sentenced on 10 December.