ZURICH (Reuters) - The Swiss competition regulator has opened an investigation into possible collusion in the precious metals market by several major banks, it said on Monday, the latest in a string of manipulation probes.

Switzerland's WEKO watchdog said its investigation, the result of a preliminary probe, was looking at whether UBS, Julius Baer, Deutsche Bank, HSBC, Barclays, Morgan Stanley and Mitsui conspired to set bid/ask spreads.

"It (WEKO) has indications that possible prohibited competitive agreements in the trading of precious metals were agreed among the banks mentioned," WEKO said in a statement.

A WEKO spokesman said the investigation would likely conclude in either 2016 or 2017, adding that the banks were suspected of violating Swiss corporate rules.

The banks face financial penalties if WEKO finds them guilty of wrongdoing, the spokesman said. He declined to comment on the size of any possible penalty from the probe.

The WEKO investigation is the latest in a long line of probes into manipulation of the precious metals and foreign exchange markets.

Last year, Switzerland's financial regulator Finma said it had found a "clear attempt" to manipulate precious metals price benchmarks during a cross-market investigation into trading at UBS.

As part of ongoing obligations imposed by Finma, UBS is seeking to automate at least 95 percent of its global foreign exchange and precious metals trading by the end of 2016.

In May, four major banks pleaded guilty to trying to manipulate foreign exchange rates and, with two others, were fined nearly $6 billion in another settlement in a global investigation into the $5 trillion-a-day market.

A Julius Baer spokesman said the bank was cooperating with authorities.

In a statement, Deutsche Bank said it was cooperating with requests for information from "certain regulatory authorities" over precious metal benchmarks but declined to comment further.

Representatives for UBS, Barclays, Morgan Stanley and HSBC declined to comment. Mitsui was not immediately available for comment.