MILAN, Oct 21 (Reuters) - Automakers have formed alliances to help them avoid hefty European Union fines on carbon emissions by purchasing credits from electric vehicle companies.

Several legacy automakers face potential fines as the transition to EVs in Europe has proved slower than expected in recent years.

Here are details about the regulation and alliances for 2025, as of Tuesday:

FINES

EU fines - which automakers have said could reach up to 15 billion euros ($17.5 billion) - were initially envisaged on 2025 carbon emission levels. In March, however, the European Commission yielded to pressure from automakers and allowed compliance based on their average emissions over 2025-2027.

SCOPE

All current alliance agreements, identified by the names of their 'pool managers', will expire at the end of this year. They are expected to be renewed in coming years.

NISSAN

Japan's Nissan 7201.T in October pooled with Chinese EV giant BYD 002594.SZ.

KG MOBILITY

Another pool was formed at the end of September by South Korea's KG Mobility 003620.KS and Chinese EV maker Xpeng 9868.HK.

TESLA

In January, Tesla TSLA.O formed a pool with Stellantis STLAM.MI, Toyota 7203.T, Ford F.N, Chinese EV maker Leapmotor 9863.HK, Mazda 7261.T and Subaru 7270.T. Japan's Honda 7267.T and Suzuki 7269.T joined the pool in March.

MERCEDES

This pool was also formed in January to include Mercedes MBGn.DE, Volvo Car VOLCARb.ST, EV maker Polestar A4N1y.F and Smart Automobile. Volvo Car and Polestar are both backed by China's Geely GEELY.UL. Geely's Chairman Li Shufu holds a 9.69% stake in Mercedes, making him the group's second-largest shareholder after China's BAIC Group 1958.HK. Smart Automobile is a joint venture between Mercedes and Geely.

EV FORECASTS

EVs made up 12% of total European light vehicle sales last year, according to consultant AlixPartners, and are expected to reach 15% this year. Their market share is forecast to increase to 24% in 2027 and to 40% by the end of the decade, according to AlixPartners.

($1 = 0.8575 euros)

(Reporting by Giulio Piovaccari and Alessandro Parodi. Editing by Tomasz Janowski and Mark Potter)

((giulio.piovaccari@thomsonreuters.com))