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Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>EuFe</mi><mn>2</mn></msub><msub><mrow><mo>(</mo><msub><mi>As</mi><mrow><mn>1</mn><mo>−</mo><mi>x</mi></mrow></msub><msub><mi mathvariant="normal">P</mi><mi>x</mi></msub><mo>)</mo></mrow><mn>2</mn></msub></mrow></semantics></math></inline-formula> (<i>x</i> = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>T</mi><mi>c</mi></msub><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, shows an extraordinarily large decrease. It starts at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>T</mi><mi>c</mi></msub><mrow><mo>(</mo><mi>H</mi><mo>=</mo><mn>0</mn><mo>)</mo></mrow><mo>≈</mo><mn>24</mn><mspace width="0.222222em"></mspace><mi mathvariant="normal">K</mi></mrow></semantics></math></inline-formula> in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>c</mi></msub></semantics></math></inline-formula> moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>c</mi></msub></semantics></math></inline-formula> to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>EuFe</mi><mn>2</mn></msub><msub><mrow><mo>(</mo><msub><mi>As</mi><mrow><mn>1</mn><mo>−</mo><mi>x</mi></mrow></msub><msub><mi mathvariant="normal">P</mi><mi>x</mi></msub><mo>)</mo></mrow><mn>2</mn></msub></mrow></semantics></math></inline-formula> superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mi>Eu</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></semantics></math></inline-formula> lattice act as efficient pairbreakers leading to a significant <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>c</mi></msub></semantics></math></inline-formula> reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>m</mi></msub></semantics></math></inline-formula> (less than 1 K) after the irradiation drives <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>c</mi></msub></semantics></math></inline-formula> to below <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>m</mi></msub></semantics></math></inline-formula>. Our results suggest that FM and SC phases coexist microscopically in the same volume.