The regulatory clock started ticking on commodities trader Glencore's takeover of mining group Xstrata after the pair said they will notify the European Commission of the proposed $90 billion (57 billion pound) deal.

Once the notification is acknowledged, the commission will have 25 days to decide whether to approve, reject or begin an in-depth probe into the plan to create the world's fourth-largest natural resource company by market capitalisation in just one of a series of antitrust hurdles the two companies will have to clear.

EU competition authorities have in the past considered the two sides as a single entity as Glencore already owns over 34 percent of Xstrata - enough to exercise control from a competition perspective, and therefore enough to avoid a merger filing and possible probe.

But lawyers and industry sources have said it was unlikely the commission would wave through the largest mining takeover deal to date. Indeed, the Commission notified the parties on Thursday that it required official notification of the merger, implying it will consider the two as separate companies and will examine the deal, instead of leaving it to individual member states.

Glencore and Xstrata said in a statement they would notify the Commission under EU merger regulations.

The parties expect the merger between Glencore and Xstrata not to result in any negative impact on competition in the commodity markets in which the two companies operate, the two sides said in a statement.

In fact, the merged firm is expected to be able to offer customers a wider range of products and services and provide improved security of supply to satisfy customer demand.

The office of Competition Commissioner Joaquin Almunia declined to comment.

We yet have to be notified officially of this operation. Until this is the case, we won't make any comment.

The deal is not expected to face major antitrust issues, but the sheer number of authorities it needs to negotiate with is likely to add to an already lengthy merger timetable.

Glencore and Xstrata combined become the world's largest thermal coal exporter, and the largest producer of both zinc and ferrochrome. But the picture is complex -- in thermal coal, Glencore and Xstrata's export capacity is around 72 million tonnes, but that is less than 10 percent of the global total -- well below the threshold deemed significant by most antitrust authorities.

(Reporting by Clara Ferreira-Marques; Additional reporting by Julien Toyer in Brussels; Editing by Mike Nesbit and Chris Wickham)