A bstract Measurements of the branching fraction ratio $$ \mathcal{B}\left(\phi \to {\mu}^{+}{\mu}^{-}\right)/\mathcal{B}\left(\phi \to {e}^{+}{e}^{-}\right) $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>B</mml:mi> <mml:mfenced> <mml:mrow> <mml:mi>ϕ</mml:mi> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> </mml:mfenced> <mml:mo>/</mml:mo> <mml:mi>B</mml:mi> <mml:mfenced> <mml:mrow> <mml:mi>ϕ</mml:mi> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> </mml:mfenced> </mml:math> with $$ {D}_s^{+}\to {\pi}^{+}\phi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mi>s</mml:mi> <mml:mo>+</mml:mo> </mml:msubsup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ϕ</mml:mi> </mml:math> and D + → π + ϕ decays, denoted $$ {R}_{\phi \pi}^s $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>R</mml:mi> <mml:mi>ϕπ</mml:mi> <mml:mi>s</mml:mi> </mml:msubsup> </mml:math> and $$ {R}_{\phi \pi}^d $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>R</mml:mi> <mml:mi>ϕπ</mml:mi> <mml:mi>d</mml:mi> </mml:msubsup> </mml:math> , are presented. The analysis is performed using a dataset corresponding to an integrated luminosity of 5 . 4 fb − 1 of pp collision data collected with the LHCb experiment. The branching fractions are normalised with respect to the B + → K + J/ψ (→ e + e − ) and B + → K + J/ψ (→ μ + μ − ) decay modes. The combination of the results yields $$ {R}_{\phi \pi}=1.022\pm 0.012\left(\textrm{stat}\right)\pm 0.048\left(\textrm{syst}\right). $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>R</mml:mi> <mml:mi>ϕπ</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1.022</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.012</mml:mn> <mml:mfenced> <mml:mtext>stat</mml:mtext> </mml:mfenced> <mml:mo>±</mml:mo> <mml:mn>0.048</mml:mn> <mml:mfenced> <mml:mtext>syst</mml:mtext> </mml:mfenced> <mml:mo>.</mml:mo> </mml:math> The result is compatible with previous measurements of the ϕ → ℓ + ℓ − branching fractions and predictions based on the Standard Model.