LONDON/RIO DE JANEIRO, Sept 15 (Reuters) - Brazil's state-run oil firm Petrobras has lifted its ban on trading with major commodity traders Vitol, Trafigura and Glencore, in place since Brazilian prosecutors announced a bribery probe in 2018, the company told Reuters.

"After a temporary suspension period with the companies cited, the company re-initiated business after adopting and perfecting a series of specific measures meant to bring more security to the commercial relationship," Petrobras said in a statement to Reuters.

Brazilian prosecutors announced in early December 2018 that they were investigating the three major energy trading firms, along with a number of smaller ones, for paying at least $31 million in bribes to Petrobras employees in exchange for sweetheart oil deals.

The prosecutors have alleged the top brass at the firms were fully aware of the alleged bribery scheme.

Petrobras said in its statement late on Monday that it had standardized and improved the monitoring of negotiations from calls to messages, instituted "Know your client" procedures and carried out integrity checks of its traders, among other measures.

Vitol, Glencore and Trafigura declined to comment.

One source familiar with the matter said the suspension had been lifted around three months ago.

After Brazilian prosecutors announced their investigation, the U.S. Justice Department opened its own probe and Swiss authorities searched the trading firms' offices in Geneva in November last year at Brazil's request.

Vitol and Trafigura have repeatedly said they have a "zero tolerance policy in respect of bribery and corruption". Vitol said it was cooperating with authorities. Glencore has previously said it takes ethics and compliance seriously and was cooperating with authorities.

Trafigura has said that "any suggestion that Trafigura’s current management knew that its payments would be used to make improper payments to employees of Petrobras is not correct." (Reporting by Julia Payne; Editing by Susan Fenton)