New laws will be needed to stop so-called "vulture funds" from using the British courts to try to undermine Greece's new debt agreement, campaigners have warned.
Vulture funds, which buy up debt from weakened entities with the hope of stripping assets when they default, are believed to have bought significant numbers of Greek bonds for a fraction of their actual value.
The funds are now holding out for full repayment.
Some bonds have been written with recourse to English law, meaning it would be London courts which hear their cases, the Jubilee Debt Campaign warned.
The group called on Chancellor George Osborne to introduce legislation making such practices illegal.
If such lawsuits are effective, they will not only take money from highly indebted Greece, but will eat into funds contributed to the controversial bail-out packages by countries like Britain.
Jubilee Debt campaign spokesman Nick Dearden said: "Greece's European neighbours have done enough damage to the Greek economy through a series of bank bail-outs.
"Now our courts could be used to squeeze the last drop of blood out of the Greek people."