Six "potentially dangerous" terror suspects - including two men connected to an airline liquid bomb plot - will be "free and unconstrained" by the end of the year, the terrorism watchdog has warned.

The suspects - who also include a man associated with four of the attempted suicide bombers involved in failed attacks on London on July 21 2005 - are subject to special measures designed to protect the public from terrorism.

But the terrorism prevention and investigation measures (Tpims), unlike the control orders they replaced, have a maximum time limit of two years, David Anderson QC, said the independent reviewer of terrorism legislation.

In December, Ibrahim Magag, who is understood to have attended terrorist training camps in Somalia, absconded from a Tpim notice after ripping off his electronic tag.

Control orders could be extended year on year without limit, while Tpims can be extended after a year for another 12 months before they expire.

In his first report on Tpims, Mr Anderson said: "Some subjects who have been judged by the Home Secretary and by the courts to be potentially dangerous will then, absent prosecution or new evidence of terrorism-related activity, be free and unconstrained."

Six Tpim subjects, out of eight in force at the end of February, will see the curbs - such as overnight residence, travel and financial restrictions - expire in January 2014.

One of the men was suspected of "would-be participation" in a failed 2006 plot to "commit mass murder by bringing down transatlantic passenger airlines by suicide bombings".

The plot led to 12 convictions and eight life sentences, but this individual was never arrested or put on trial and another Tpim subject was acquitted of involvement in the same terror plot. Another of the men was a train driver considered to be a UK extremist seeking to obtain terrorist training in Pakistan.

One of the subjects attended a terrorist training camp in Cumbria in 2004 in the company of four of the five attempted suicide bombers involved in failed attacks on London in July 2005.