The document assessed the options for power reactors for a new generation of submarinesThe Ministry of Defence has admitted that secret information about the UK's nuclear powered submarines was made available on the internet by mistake.
A technical error meant blacked out parts of an online MoD report could be read by pasting into another document.
Details were reported to include expert opinion on how well the fleet could cope with a catastrophic accident.
The MoD said a secure version had now been published and it was working to stop such an incident happening again.
Information also included measures used by the US Navy to protect its nuclear submarines, the Daily Star Sunday reported.
Declassified dossierThe document was an assessment by the head of the Defence Nuclear Safety Regulator, Commodore Andrew McFarlane, of the options for reactors for future submarines to replace the Trident fleet.
Some parts of the declassified dossier, published on the Parliament website, were redacted to protect sensitive information.
In the sections that were supposed to be released, it was concluded that existing reactors were "potentially vulnerable" to fatal accidents.
This caused ministers to suggest that different types would be used in future.
'Public domain'Former Army officer, Conservative MP Patrick Mercer, said the leak was "potentially catastrophic", with the details "hugely interesting" to the UK's enemies.
A spokesman for the MoD, which was told about the blunder by the Daily Star Sunday, said: "The MoD is grateful to the journalist for bringing this matter to our attention.
"As soon as we were told about this, we took steps to ensure the document was removed from the public domain and replaced by a properly redacted version.
"We take nuclear security very seriously and we are doing everything possible to prevent a recurrence of this."
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The BBC's Michael Voss in Havana says Raul Castro realises "they made real mistakes out of idealism"
Cuban President Raul Castro has said top political positions should be limited to two five-year terms, and promised "systematic rejuvenation" of the government.
President Castro was speaking at the start of the first congress of Cuba's ruling Communist Party in 14 years.
He said the party leadership was in need of renewal and should subject itself to severe self-criticism.
The proposal is unprecedented under Cuban communism.
Mr Castro, 79, made clear the limits would apply to himself.
He took over from his brother Fidel in 2008 and between them they have ruled Cuba for 52 years.
He acknowledged that "the confidence of the majority of Cubans had been tested, with regard to the party and the revolution".
Cubans, he said, would have to overcome a "mentality of inertia" and said the only thing that could threaten the revolution was "our inability to rectify errors".
Former leader Fidel Castro, who missed the day's events, said he regretted his absence but felt proud of the congress and preceding military parade.
Economic changeIn his speech, he said the limits of two consecutive five-year terms would apply to "the current president of the Council of State and his ministers" - a reference to himself.
Much of what he said concerned his plans to reduce the role of the state in the economy and encourage private enterprise.
It would, he said, take at least five years to update Cuba's economic model.
Free education and healthcare would still be guaranteed, but mass subsidies of basic goods would be removed and social spending would be "rationalised".
President Castro said 200,000 people had already registered as self-employed since the changes were announced last October, doubling the number of Cubans working for themselves.
But he insisted the socialist character of Cuba would be "irreversible" and accumulation of property would not be allowed.
The Communist Party Congress was preceded by a one of the largest military parades Cuba has seen in years, marking the 50th anniversary of the failed Bay of Pigs invasion by US-backed exile groups.
Troops and armoured vehicles paraded through Havana's Revolution Square, followed by hundreds of thousands of civilians waving flags and chanting slogans.
The parade and Congress also celebrate 50 years since Fidel Castro proclaimed that his was a socialist revolution.
In 1961, Cuban exile groups armed and trained by the American CIA came ashore at the Bay of Pigs, 100 miles (160 km) southeast of Havana, in an attempt to overthrow him.
But after 72 hours of combat the invasion was defeated, in what Cuba celebrates as the "first great defeat for US imperialism in Latin America".
Raul Castro, who is a general, donned military fatigues for the parade.
'Feeling of pride'There was no sign of his brother, ex-President Fidel Castro, who is now 84 years old.
But remarks attributed to the former leader later appeared on the Cubadebate website, in which he spoke of a "feeling of pride" at the day's events.
"It's worth to have lived to see that show today and it's worth always to remember those who gave their life to make it possible," he was quoted as saying.
He said he could not have coped with the heat for more than an hour, but regretted being unable to attend.
"Believe me, I felt myself ashamed when I saw some you looking for me at the tribune," he said.
"I thought you would understand I can no longer do any more what I've done many times."
The four-day party congress is expected to see 1,000 delegates back all or part of a package of nearly 300 reforms.
It will also elect a new 100-member Central Committee and 19-member Politburo and 10-member Secretariat.
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