The launch was delayed for six days on 4 August after a fault was discovered during testing of Proton's Breeze M upper stage.
The delay was necessary to allow time to replace and retest an electrical component.
The Proton M will carry the Inmarsat-4 F3 into geostationary orbit at 35,786km (22,240 miles). Once tested and operational, the I-4 F3 will provide satellite phone and mobile broadband services to North and South America at 98 degrees west.
And together with its two sister satellites, already in orbit, the completed constellation will deliver full and seamless global coverage for Inmarsat's entire range of broadband mobile satellite services.
You can view the launch live from Kazakhstan from 22:18 UTC (23:18 London time) on 18 August. Just follow the link provided from the F3 launch graphic on the homepage at www.inmarsat.com.