McLaren's engine partner Honda says the two-time champion will receive a five-place grid drop for using too many examples of a specific engine part.
Honda expects further penalties when it finalises its plans for the race after Friday practice.
As Alonso is likely to qualify on the cusp of the top 10, a back-row start appears inevitable.
Honda, struggling with performance and reliability, is keen to try to avoid penalties at the Hungaroring in two weeks' time because it expects it to be one of the most competitive races for McLaren.
"It will not change too much if we take a small penalty or a big one," Alonso said. "Maybe it is better to take the big one and arrive in Hungary with a little bit more safety."
Hungary is one of the tracks where engine performance is least important in overall pace, whereas Honda will struggle at Silverstone because of the high-speed nature of the track and the fact that the lack of braking around the lap exposes the engine's relative lack of effectiveness of hybrid energy recovery and deployment.
Alonso and team-mate Stoffel Vandoorne had already amassed a total of 110 grid-place penalties even before the Silverstone weekend as Honda struggles to get to grips with F1 technology.
The first engine part that has been changed on Alonso's car for this weekend is the energy store - or battery - of his hybrid system, of which Alonso has already used the maximum permitted number of four this season.
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Alonso will be using the newest-spec internal combustion engine - known as Phase Three.
This new engine also included a new-spec MGU-H - the part of the hybrid system that recovers energy from the turbo - aimed at solving a weakness in the bearing that has led to a series of failures this season.
But this new-spec MGU-H failed on Saturday morning in Austria and Alonso had to have a Phase Two specification engine fitted for the rest of the weekend because there was not enough time to fit an old MGU-H to the new-spec engine.
Alonso will start the Silverstone weekend with a Phase Three internal combustion engine fitted with a Phase Two MGU-H and Honda has yet to reveal what other changes it will make after that.
Fernando Alonso is poised to start the British Grand Prix from the back of the grid following fresh engine penalties.