PARIS (Reuters) - Building materials groups Holcim (>> Holcim Ltd) and Lafarge (>> LAFARGE) said on Tuesday they had formally notified the European Commission of their merger plan to create the world's biggest cement group.

The move kicks off official scrutiny in Europe of a deal that was designed from the start to be accompanied by billions of dollars worth of asset disposals in order to secure regulatory approval worldwide.

The Commission will decide on the deal by Dec. 15.

The all-share combination, which requires approval from regulators around the world, already has the green light from seven countries, executives told reporters on a conference call.

In a statement, the partners who plan to create the group with over $40 billion (24.79 billion pounds) in annual sales also said they had adjusted slightly their European disposals plan after "constructive pre-notification discussions" with the Commission.

The pair plan to keep Lafarge's Mannersdorf plant in Austria, and divest all of Holcim's assets in Slovakia.

They said they were in ongoing talks with buyers of assets, and that they still plan to close the deal, which was announced in April, during the first half of next year.

On a conference call with reporters, Lafarge CEO Bruno Lafont and Holcim CEO Bernard Fontana would not be drawn on the value of what was for sale or the prices being offered, but Lafont said the assets represented around 12 percent of combined annual sales.

Combined sales in 2013 were about 31.5 billion euros($40.05 billion).

Lafont said the pair were talking to both financial and industrial buyers.

Several people familiar with the matter told Reuters earlier in October that Irish cement maker CRH (>> CRH PLC) had teamed up with Mexican rival Cemex (>> Cemex SAB de CV) to explore a bid for all the assets Lafarge and Holcim plan to sell.

Germany's HeidelbergCement (>> HeidelbergCement AG) and Brazilian firm Votorantim Cimentos SA are also considering a joint bid for the entire portfolio, the sources said.

These industry consortia would compete with several private equity groupings that have been formed to pursue a deal for the assets, which could be valued at anywhere between 4 billion and 7 billion euros ($5-8.85 billion), they said.

In all, the pair made some 20 notifications regarding the merger. So far they have received approval from Russia, Ukraine, Turkey, South Africa, Morocco, Kenya and Singapore.

(Reporting by Andrew Callus; Editing by Anand Basu and Jane Merriman)

Stocks treated in this article : LAFARGE, HeidelbergCement AG, CRH PLC, Holcim Ltd, Cemex SAB de CV