Foreign Secretary David Miliband has warned that the danger of a terrorist attack "remains very real" and Britain should be "extremely vigilant".
His remarks came hours after an audio tape said to be from al Qaida leader Osama Bin Laden claimed responsibility for the Christmas Day airline bombing attempt in Detroit.
Mr Miliband said the attack demonstrated "the links that can exist between different terrorist groups".
But he told BBC One's Andrew Marr Show that he would wait to see whether the tape was authentic.
"Let's wait to see what he actually says; we know that the al Qaida senior leadership are in the badlands of the Afghan-Pakistan border, probably on the Pakistan side," he said.
"We know too that the Detroit attack was the first time that al Qaida of the Arabian Peninsula, which is a sub-group of the al Qaida franchise ... represents an attack on the West rather than an attack within the Middle East.
Mr Miliband said he believed that "a significant part of the al Qaida senior leadership" had been killed.
"But the danger remains very real," he said.
"You've seen this week that the Home Secretary has thought it right to raise the threat level back to the level it's been for most of the period since 9/11. It remains the case that we need to be extremely vigilant."
Alan Johnson upped the threat level from "substantial" to "severe" on Friday night - meaning an attack is "highly likely".
