* CDP group to come up with final offer in 10 weeks

* Has valued Autostrade at 8.5-9.5 bln euros - sources

* Atlantia has signalled initial proposal insufficient (Adds TCI's comment on valuation, background)

ROME/MILAN, Oct 28 (Reuters) - A consortium led by Italian state lender CDP pledged on Wednesday to come up with a final offer to buy Atlantia's 88% stake in a motorway business in 10 weeks, while repeating an initial valuation that Atlantia has signalled is insufficient.

The negotiations are part of a long-standing wrangle over Autostrade per l'Italia after a bridge it ran collapsed in 2018, killing 43 people.

The Italian government has repeatedly threatened to strip the motorway business of its operating licence, while Atlantia has sought to defend a key asset.

In its latest proposal on Wednesday, CDP said the purchase of Atlantia's stake in Autostrade would be carried out in stages and that it would update Atlantia on the asset's valuation four weeks after the start of due diligence.

The state lender, with co-investors Macquarie and Blackstone, valued the whole of Autostrade at 8.5 billion-9.5 billion euros in an initial offer, sources told Reuters.

Atlantia, controlled by the Benetton family, has said it wants a quick deal at a fair price or it will fall back on previous plans to spin off its Autostrade stake. Atlantia's shareholders meet on Friday to discuss the spin-off option.

Activist fund TCI and Spinecap fund, two minority investors in Atlantia, say Autostrade's value is higher and have both filed a complaint with the European Commission against the way the Italian government has handled the issue so far.

"We think that 88% (of Autostrade) has a value of between 11 and 12 billion euros," TCI executive Jonathan Amouyal told Frankfurter Allgemeine Zeitung on Wednesday.

That means the fund's valuation for the whole of Autostrade currently differs from that of the CDP-led consortium by around 4 billion euros.

Atlantia's board will meet later on Wednesday to examine the latest offer. Last week, it said terms in the preliminary proposal were "not yet compliant and suitable for ensuring a fair market valuation of its Autostrade stake".

Sources have said the preliminary valuation of Autostrade will also be adjusted to take into account future damage claims stemming from the bridge disaster, and the impact of a less generous tariff system laid out by the Italian Transport Authority.

CDP, which said other Italian investors could come on board in the deal, added it would own 40% of the company bidding for Autostrade, while Macquarie and Blackstone would initially hold 30% each.

It said the consortium could buy the whole of Autostrade should minority shareholders Germany's Allianz, EDF Invest, DIF and China's Silk Road fund decide to exercise their right to sell shares.

($1 = 0.8461 euros) (Writing by Francesca Landini; Editing by Tom Brown, Mark Potter and Jan Harvey)