MADRID, Jan 16 (Reuters) - Shareholders in Heathrow, Britain's largest air terminal, accounting for a combined 35% stake, intend to exercise 'tag along' rights to sell their shares alongside Ferrovial, the Spanish infrastructure operator said on Tuesday.

Ferrovial, which is in the process of selling its 25% stake in Heathrow to private equity fund Ardian and Saudi Arabia's Public Investment Fund (PIF), did not identify the Heathrow shareholders ready to exit Heathrow.

Ferrovial reached an agreement two months ago to sell its stake for $3 billion to PIF and Ardian.

Heathrow Airport Holdings Limited is owned by a consortium led by Ferrovial, while the Qatar Investment Authority owns 20%, Canadian pension fund Caisse de depot et placement du Québec hold 12.62%, GIC 11.20%, Australian Retirement Trust 11.18%, China Investment Corporation 10% and Universities Superannuation Scheme also 10%.

($1 = 0.7895 pounds) (Reporting by Inti Landauro and Andres Gonzalez; editing by Jason Neely)