Former Tory MP Rupert Allason, who also uses the alias Nigel West when he is writing about espionage, said: "There is no such thing as a completely safe encryption system."
Downing Street said it believed the theft from a MI5 officer was "opportunistic" and that there was no threat to national security.
However, he said MI5 had reviewed its procedures for moving sensitive information in the light of what had happened.
"There are strict procedures for the moving of classified material. They have been tightened since this incident," he said.
And he indicated that the MI5 officer involved could face disciplinary action for the security lapse.
The laptop was snatched at Paddington Underground station in central London.
Its data was encrypted and security officials are thought to be confident it could not be accessed.
Former Tory MP Rupert Allason, who also uses the alias Nigel West when he is writing about espionage, said: "There is no such thing as a completely safe encryption system."
"The hope is that first of all, the thieves would not have access to the kind of facilities that would be needed to decrypt that system.
"And secondly, by the time somebody did gain access to the material, appropriate steps would have been taken to make sure that it was no longer current."
But former MI5 officer, David Shayler, who is now living in exile in Paris, said the chances of a private individual being able to decrypt the information were "zero."
"The sun is more likely to melt," he said.
But he called for a full on-the-record statement about the circumstances of the theft.
"We should not take MI5 statements at face value," he said. "What they would do in these circumstances is try to play this down as much as possible. We need to be absolutely sure that all the information on this computer has been properly encrypted."
'Agencies hammered'
Tom King, who chairs the parliamentary intelligence and security committee, said he expected to receive a report on the incident, and the committee would decide whether it wanted to make any further inquiries.
"Any breach of security is serious. We have continually hammered the agencies on the importance of security at all times," he told the BBC Radio 4 Today programme.
The thief snatched the computer and fled with the agent in pursuit. The incident happened on 4 March.
MI5 is now in the front line of Britain's intelligence-gathering operation in Northern Ireland after taking extra anti-terrorist responsibilities in recent years.