Looks to be a Jacono copy of the Ace D-103 cover, credited to Ed Valigursky.
"The Invaders Left the World Facing the Deadliest Peril in its History"
Fanthorpe’s “Doomed World,” as described on the back cover:
“George Mallory was out for a quiet day’s shooting. A typical countryman in typical English country. His day’s sport was interrupted by the be beginning of the greatest catastrophe in man’s history – an alien space ship was crashing at his feet.
“The ghastly monstrosity that emerged was so hideously repulsive that no one would have guessed at the degree of intelligence and potential friendliness in its strange mind.
“Mallory shot first and asked questions afterwards. With its dying strength, the alien cursed the earth with a scientific horror beyond the comprehension of man, a horror that turned the beasts against us.
“The only escape seemed to be out in space . . . but the devastating effect of the cosmic rays, wrought havoc in the minds of the space men and the lunar expedition turned upon itself in deadly carnage.
“What would be the outcome of the terrible conflict between man and beast?
“Doomed World is a masterly authentic science fiction thriller from the able pen of R. Lionel Fanthorpe, M.B.I.S.
“It needs no further recommendation.”