The warning from Daphne Park, who served for 30 years as a senior controller for MI6, the Secret Intelligence Service, came as the parliamentary power struggle over the identity cards bill dragged on.
The House of Lords once again defeated the government last night. Peers backed a joint Conservative and Liberal Democrat amendment that would prevent ministers making identity cards compulsory until at least 2011.
It is the fourth time the Lords have sent the bill back to the Commons in a parliamentary ping-pong game that could go on for months.
At issue is the "voluntary" nature of the proposed card. Labour ministers insist that the cards will be optional, but also want to create a rule meaning that anyone renewing a passport after 2008 must also buy an identity card.
Opponents argue that is "compulsion by stealth" and question Labour's mandate for the scheme. The Labour manifesto at last year's election promised the cards would be "voluntary at first".
Baroness Scotland, speaking for the government in the Lords, insisted that MPs have already voted for the document link and that should stand.
"The Commons did discuss this issue," she said. "This is not compulsion by stealth as has been suggested."
While ministers have hinted at using the Parliament Act to force the ID cards bill through the Lords, peers insist they will continue to resist.
The former MI6 agent will be a key part of that resistance.
Baroness Park, who was made a peer by Margaret Thatcher, passed a withering verdict on the proposed cards, ridiculing ministers' suggestions that the system will make people safer. In fact, she said, the complete opposite is true.
"The very creation of such an enormous national identity register will be a present to terrorists; it will be a splendid thing for them to disrupt and blow up," she said.
"It will also provide valuable information to organised crime and to the intelligence services of unfriendly countries. It will be accessible to all of these," she said.
The warning about the risk of foreign spying comes at a time when MI5, the domestic security service, has cut its counter-espionage budget, prompting concerns among MPs who oversee the UK intelligence services.
Baroness Park concluded: "I find it extraordinarily difficult to believe why anyone would voluntarily and enthusiastically come forward and say: 'Do let me join this dangerous club'."
Baroness Park is not the first former intelligence officer to question the value of a national ID card. Dame Stella Rimington, the former head of MI5, last year said she did not believe the cards would make Britain any safer from terrorist attack; they would quickly be copied, she said.
Whereas Dame Stella's background was in combating internal threats, especially the IRA, Baroness Park has extensive experience of foreign intelligence operations.
Now in her eighties, her career began during the Second World War, when she volunteered for service and was assigned to train Resistance fighters who were being parachuted into France in the run-up to D-Day.
When the war ended, she was posted to Berlin where she was ordered to locate and secure German military scientists before they fell into the hands of advancing Russian forces.
That work brought her to the attention of MI6, who made extensive use of her services using the Cold War.
Although she has repeatedly refused invitations to publish her memoirs, many of Baroness Park's exploits as a spy are in the public domain - she is something of a living legend in the British intelligence community.
Among her Cold War postings were extensive service in Moscow, running agents inside the Soviet regime. During the Vietnam war, she was a covert operative in Hanoi.
She is also known to have been in Congo during the turbulent 1960s, at one point smuggling a defecting official out of the country in the boot of her car.