  We propose a new mechanism of (geometric) moduli stabilisation in type
IIB/F-theory four-dimensional compactifications on Calabi-Yau manifolds, in the
presence of 7-branes, that does not rely on non-perturbative effects. Complex
structure moduli and the axion-dilaton system are stabilised in the standard
way, without breaking supersymmetry, using 3-form internal fluxes. K\"ahler
class moduli stabilisation utilises perturbative string loop corrections,
together with internal magnetic fields along the D7-branes world-volume leading
to Fayet-Iliopoulos D-terms in the effective supergravity action. The main
ingredient that makes the stabilisation possible at a de Sitter vacuum is the
logarithmic dependence of the string loop corrections in the large
two-dimensional transverse volume limit of the 7-branes.
