It comes after the Government Office for the South West (GOSW) announced a further delay to the regional spatial strategy (RSS) to do more work on sustainability appraisals as a result of successful court action against the East of England Plan.
GOSW has advised planning authorities that until the plan is finalised, significant weight should be given to the secretary of state's proposed changes.
Francis Taylor Building barrister Gregory Jones responded: "Any decision-maker giving such weight to the proposed changes is open to High Court challenge. What the government office is proposing would drive a coach and horses through the strategic environmental assessment directive."
Challenges could be made to decisions which rely on the draft RSS but also the advice given to councils, Jones added. Several decisions made on appeal by the secretary of state have cited giving significant weight to the proposed changes.
Burges Salmon planning partner Gary Soloman agreed that councils following the advice may be open to court action. "It is premature for the secretary of state to be issuing such advice when he is not yet satisfied that the proposals are the most sustainable for the region."
It is not clear whether any authorities will seek to challenge the advice. Plymouth City Council wrote to GOSW after an earlier delay to the RSS, raising concerns about referring to the document in planning decisions. South Gloucestershire Council has also opposed the advice, stating that the process used to prepare the RSS is flawed.
But Barton Willmore partner Simon Prescott said: "The RSS may not be issued before the general election and the Tories have pledged to abolish regional planning policy. My concern is that councils will use this delay to avoid making decisions."