The five, who have not been charged, are among 25 people arrested in connection with the alleged plot since police carried out pre-dawn raids on August 10.
Five have since been released without charge, and 15 have been charged.
Of the 15 who have been charged, 11 face the most serious charge of conspiracy to murder and preparing acts of terrorism, while the other four face lesser charges.
Under British anti-terror laws, police can hold suspects for up to 28 days without charging them with an offence, subject to regular court approval.
The five still in police custody have so far been detained for 21 days without charge, the longest for any suspect since the new anti-terror legislation was passed by the House of Commons last November.
"This evening Anti-Terrorist Branch officers were granted warrants of further detention for five people who remain in custody in connection with an ongoing anti-terrorist operation," a police spokesman said.
"The warrants allow the Metropolitan Police Service to detain them for a further seven days, until September 6, 2006."
Security at British airports was stepped up to unprecedented levels in the aftermath of the police raids, and the national threat level ramped up to "critical" -- the highest of five levels.
The suspects were allegedly planning to smuggle seeming innocuous liquids on to planes with the intention of assembling them into bombs on board.
The alleged plot was described by one senior police officer as "an attempt to commit mass murder on an unimaginable scale".
At least two of those freed had been on a list of 19 people whose names were published and whose assets were frozen by the Bank of England, Britain's central bank. The assets of all 19 have also been frozen.