The Association for the Protection of Rural Scotland lobby group said it supports plans to extend the Pentland Hills Regional Park into the Scottish Borders and South Lanarkshire.
It comes after the move by SNP MSP Christine Grahame to push the boundaries of the current protected area covering the Pentlands, south of Edinburgh.
The campaign group said the new lines reflected the original plan for the park.
In its response the group said: "It is clear that the compromise boundary established in 1986 lacks logic, as it resulted from administrative considerations at the time rather than defensible landscape criteria, so it is right that this anomaly should now be rectified".
The group said: "This will help the entire Pentland Hills range and its surrounding communities to achieve its full potential, and will secure the integrated management of the Pentland Hills for the benefit of future generations."
John Mayhew, APRS director, added that extending the boundary would provide greater opportunities for a wider range of people to enjoy the health benefits of outdoor recreation.
Regional parks are usually funded by neighbouring councils, but do not have the same planning restrictions as Scotland's two national parks, Cairngorm and Loch Lomond and the Trossachs.
The two other regional parks are Clyde Muirshiel, near Lochwinnoch in Renfrewshire and Lomond Hills in Fife.