According to the letter sent by Clifford Chance, the Trust has not provided 'vital market information and analysis' to the NS and changed the schedule for the PVT without consulting the organisation.
The society is asking for a suspension of the current market impact and public value tests being carried out by Ofcom and the BBC until this information has been given out, a press release from the group said.
The proposals by the BBC would see £68 million invested in local news video across a network of sites covering 60 UK regions.
The plans are currently subject to a public value inquiry by the Trust – the outcome of which is expected on November 27, but have met with opposition from regional newspaper publishers and the NS.
NS has also called for the suspension of the BBC's application to the Trust and for an investigation by the Secretary of State into the body's handling of the plans and the PVT.
"The BBC Trust cannot be the chief cheerleader for the BBC, encouraging it to extend local services out of more and more taxpayers' money, at the same time as being the independent regulator determining the public value of those services and their impact on local media," said David Newell, NS director, in the release.
"Our legal advice confirms that the Trust and Ofcom are acting in an inappropriate and unlawful manner."
The society's legal team has also criticised comments made by BBC Trust chairman Michael Lyons about the state of the UK's local media, saying that such remarks are 'difficult to reconcile (...) with the conduct of an objective PVT'.
"There has been a backlash from publishers and commentators to the remarks, which the NS and its members fear show that the Trust has already pre-judged the decision to approve the local video proposal before the conclusion of the PVT," said the release.
Clifford Chance has asked for a response to the letter from the BBC Trust and Ofcom by November 14.
UPDATE 4/11/2008 - The BBC Trust has responded to Journalism.co.uk with this statement:
"The public value test is an independent, evidence-based process used to evaluate significant service proposals from the BBC. It is the decision-making process for new services set out in the Royal Charter and Agreement.
"The publication in November of the Trust's provisional conclusions on BBC management's proposals for local video services will be subject to public consultation. We encourage stakeholders to participate in this process so the Trust can take all opinions into account. The Trust will not reach a final decision until February 2009.
"The BBC Trust and Ofcom have met with the Newspaper Society on several occasions to discuss their concerns. Consistent with that approach, we will also respond to their letter shortly and before the Trust reaches its provisional conclusions, the deadline for which is 27 November."
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