Islanders in Guernsey, who oppose plans to fill in the Sunken Gardens, say they will not give up without a fight.
Samantha Gillott, who is behind an online campaign group, says they will use the Clameur de Haro if needs be.
The ancient legal injunction, although seldom used, is fully enforceable in Guernsey and Jersey law.
But those opposed to the plans are hoping legal action will not be necessary.
A protest has been organised for Sunday in an attempt to halt the proposed redevelopment.
Samantha Gillott told ITV reporter, Charlie Frost, that the green space is vital.