Twenty-nine people died as a result of the Omagh bomb
A powerful Westminster committee has again called on the government to allow it access to a report surrounding the Omagh bombing.
The NI Affairs Committee wants its chairman to see the report on telephone intercept intelligence.
The committee said the government was "being unreasonable" in not allowing Sir Patrick Cormack to read the report.
It said the government had failed to respond adequately to concerns about the limits of the inquiry.
Downing Street gave its response last month to the committee's report on the 1998 Omagh bombing in which 29 people died.
Defending the government's refusal to share intelligence over the bombing, the prime minister's official spokesman said it was about national security and only a limited number of people could have access to sensitive information.
MPs had wanted the information as part of an inquiry into how much intelligence services knew about the bombers.
Welcoming the government's quick response to his committee's report, Sir Patrick Cormack said: "We cannot overlook what we consider to be the serious deficiencies of this response.
"We are deeply disappointed that the government has failed properly to address the recommendations in our report. We strongly urge our successor committee and the next secretary of state to consider again the points made in our report."
The report, published on 16 March, said too many questions remained unanswered.
The committee had called for a fresh investigation into whether the state withheld vital information from detectives hunting the Omagh bombers.
No-one has been convicted of the murders.
Two men were tried and acquitted on offences related to the bombing. Colm Murphy was tried in Dublin in 2002 and retried earlier this year and Sean Hoey was tried in Northern Ireland in 2007.
The NI Affairs committee undertook the inquiry into the security services' role after claims in a BBC Panorama documentary that the government's listening station GCHQ had monitored suspects' mobile phone calls as they drove to Omagh from the Irish Republic on the day of the atrocity.
As well as calling for a fresh examination of the intelligence, the committee's report found that questions remained about whether the bombing could have been pre-empted by action against terrorists who carried out earlier bombings in 1998.
It called for a definitive statement on whether the names of those thought to have been involved in the bombing were known to the intelligence services, Special Branch, or the RUC in the days immediately after the bombing, and if so, why no arrests resulted.
The committee asked the government to justify the argument that the public interest was best served by keeping telephone intercepts secret rather than using them to bring murderers to justice.
It also called on the UK's Intelligence and Security Committee to reconsider how any intercept intelligence was or was not used.
The committee also recommended that the government considered providing legal aid for the victims of terrorism if they brought civil actions against suspected perpetrators once criminal investigation has failed to bring a prosecution.