Chancellor Angela Merkel's conservative-led coalition is coming under mounting pressure to call off a secret arms deal involving the sale of 200 German Leopard tanks to Saudi Arabia.

Opposition Social Democrats, who have denounced the deal as "politically and morally indefensible", insisted yesterday that details of the sale be put before parliament and pledged to mount a concerted campaign against the project.

News of the deal, which was reached by Germany's security council without any details being made public, was leaked to Der Spiegel magazine last week, provoking widespread criticism from MPs including members of Ms Merkel's party.

Opponents point out that Leopard tanks are advertised as being highly versatile and suitable for urban terrain and could easily be used by the Saudi government to crush dissent.

Volker Rühe, a former conservative defence minister, demanded the sale be abandoned.