The BBC reported on Wednesday that former British Prime Minister Tony Blair was warned in the aftermath of 9/11 that the UK could suffer a terrorist incident “more horrific” than the World Trade Center attack.

Recently declassified documents show that Blair warned in a letter from former Defense Secretary John Stanley that terrorists might use a dirty bomb to target Britain.

Stanley, who served under former Prime Minister Margaret Thatcher, said the government should act on the assumption that one or more British cities will be struck by weapons of mass destruction “in the near and foreseeable future,” The Times reported.

“I don’t need to tell you that if terrorist organizations are successful in using nuclear, chemical or biological weapons of mass destruction in urban areas, the number of deaths, the length of time it takes a large number of people to die, and the degree of extreme pain,” he added in the letter sent days after the events. 9/11 “that the suffering of the victims will be far greater than anything we have seen in New York or Washington this week.”

Stanley told Blair that it was “not in the interests of national security” to publish the warning, keeping the matter secret within the government.

Although Blair immediately strengthened precautionary measures at airports and government buildings across the UK after 9/11, Stanley said authorities should focus on measures to deal with weapons of mass destruction, including stockpiling vaccines and anti-radiation supplies.

Stanley also proposed expanding government oversight of potential terrorists, and relaxing detention regimes.

The series of declassified letters also reveal Blair’s reluctance to act against the coalition formed in the aftermath of 9/11 by launching an immediate invasion of Iraq.

The 2001 anthrax attacks in the United States, which occurred shortly after 9/11, proved another controversial point.

At the time, it was suspected that the attacks were carried out with motives similar to the World Trade Center strike, either by al-Qaeda or Saddam Hussein’s Iraq, although seven years later, an American microbiologist was named the sole culprit. But Blair in particular wondered whether the anthrax had been produced in Iraq.

Jonathan Powell, Blair’s chief of staff, said in a note: “We had to be careful about making allegations. He (Blair) would not mind dealing with Saddam Hussein at the appropriate time.”

But we must not undermine the coalition by rushing to confront Iraq now. The Islamic world was very fragile and we might lose France and Germany as well.”

The letters also show correspondence between Thatcher and Blair, with the former praising the latter for his support of the United States in the wake of 9/11, The Guardian reported.

In one handwritten note dated 4 April 2002, Thatcher said: “You will find, as I have, that as soon as one international crisis recedes, another soon threatens.

“I greatly admire the resolve you display. I have made sure that Britain is known as a staunch defender of liberty and a staunch ally of America. This is the best reputation our country can have.”

Stanley’s concerns about a possible deadly terrorist attack on Britain appear to have been taken seriously by the Blair government, taking action behind the scenes to strengthen counter-terrorism measures.

David Omand, the former coordinator for security and intelligence in the Cabinet, said changes were often made in secret, due to concerns that the government might “put people in a state of fear”.