Thomas could order the information's release on public interest grounds, raising the prospect of a government veto.
The paper said the outcome would play a big part in determining public confidence in the new information law.
"If ministers effectively castrate the commissioner at the first test of his powers, public confidence in the value of the (law) is likely to plunge," it said.
"But if the commissioner shirks the challenge, he too will be likely to face criticism."
The commissioner's office declined to comment on individual requests yet to be made under the new law.
Blair's Downing Street office also declined comment and cited what it said was convention not to disclose the advice on the legality of the war given to Blair by Attorney-General Lord Goldsmith.
The Guardian has failed in previous attempts to unearth the advice, which it believes says British involvement in Iraq was legally questionable.
The prime minister has faced sustained pressure over Iraq and his close support of US President George W. Bush, much of it from within his own Labour party.
Though the policy has cost him popular support, Labour is still tipped to win an election expected in early 2005.
Aside from Iraq, though, information campaigners were keen to play up the new law's benefits.
They said it would improve debate on controversial public safety issues and perhaps throw light on past ones such as a mass outbreak of the brain-wasting mad cow disease and arguments about planting genetically modified crops.
The long-delayed legislation, which comes into effect decades after similar moves by many rich nations and a couple of centuries after Sweden, includes a strong public interest clause to override exemptions.
In addition to government, the law also requires regulators, local authorities, publicly owned companies and parliament itself to respond to public requests for information.
In total, some 100,000 public authorities are affected according to Information Commissioner Thomas.
"It's all about sweeping away unnecessary secrecy," he told BBC Radio on Friday. "For too long, we've lived in a culture of official secrecy - those governing us wanting to keep information to themselves."
The opposition Conservative party has accused government officials of shredding unusually large numbers of documents in anticipation of the law, something Thomas said was not proven.
"As from tomorrow it becomes a criminal offence to do that and I've seen no hard evidence of that going on," he said.