MILAN (Reuters) - UniCredit will seek ECB clearance to potentially buy up to 30% of Commerzbank, three people close to the matter said, in order to have the "flexibility" the Italian bank's CEO Andrea Orcel said he needs to plan his next move.

UniCredit emerged as the biggest private investor in Commerzbank after it outbid rivals in a tender to buy a 4.5% stake from the German government. UniCredit had previously bought another 4.5% stake on the market.

The European Central Bank vets investors who want to own at least 10% of a bank, to ensure they are a suitable shareholder. The same applies for thresholds of 20%, 30% or 50%.

UniCredit's 9% holding in Commerzbank, which has been a takeover target for the Italian lender for more than two decades, could lead to a full takeover, Orcel has said, provided there is broad support for a deal that will "create added value for Commerzbank, UniCredit, Germany and Europe."

UniCredit could also just keep the current stake or sell it, Orcel has said. In disclosing the stake, UniCredit said it would seek ECB approval to potentially raise it above 9.9%, so that it could then move "if and when necessary".

UniCredit plans to seek approval to potentially acquire a stake of up to 30% because that is the threshold that under German rules triggers a mandatory buyout offer, the three people close to the matter told Reuters.

The filing is expected in the next few days, they added.

Il Messaggero daily first reported news of the up to 30% threshold.

UniCredit will apply to Germany's financial regulator BaFin, which will then send on the request to the ECB. The ECB has up to 60 days to reply, which can be extended to 90 if necessary.

Investors normally apply for successive thresholds, triggering a separate authorisation process every time.

By seeking clearance to increase the Commerzbank stake to just below the mandatory buyout offer threshold, UniCredit will secure the widest room for manoeuvre in the shortest timeframe.

"We want the flexibility, because as a private investor, at the moment, we may go up, we may go down, and we may combine," Orcel told Bloomberg television last week. "Unless we ask for the authorisation first, we don't have that flexibility."