Tim Cook began his first day as Apple chief executive on Thursday by assuring employees that he will stick to the winning ways of his legendary predecessor Steve Jobs.

"I want you to be confident that Apple is not going to change," Cook said in an internal email spreading across the Internet after first appearing at technology news website Ars Technica.

"Steve built a company and culture that is unlike any other in the world and we are going to stay true to that -- it is in our DNA," the message promised.

Cook referred to joining Apple 13 years ago as the best decision in his life and said he shared Jobs's optimism that even better days are ahead for the California-based firm.

"Steve has been an incredible leader and mentor to me, as well as to the entire executive team and our amazing employees," Cook told employees.

"I love Apple and I am looking forward to diving into my new role," he added.

Jobs, 56, resigned as Apple on Wednesday and became chairman of the board.

He did not give reasons, but his battle in recent years with pancreatic cancer and gaunt appearance made it likely that the decision was driven by .

"I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know," Jobs said in his . "Unfortunately, that day has come."