Lakhvi, currently lodged in Rawalpindi's high-security Adiala Jail, has been using mobile phones to contact other members of the banned LeT, Western diplomatic sources said.
The contacts have been intercepted by many intelligence agencies, including those of the US, the sources told PTI.
These contacts had been going on for "many months", said a source familiar with the tracking of Lakhvi's clandestine communications by intelligence agencies.
US officials alerted their Pakistani counterparts about the intercepts and asked them to "shut down" the contacts but no action had been taken so far by officials at Adiala Jail or other authorities, two officials of two different countries, who did not want to be named, told PTI.
"These intercepts show Lakhvi remains a key player in the affairs of the LeT despite being in custody for over two years," said an official.
US intelligence agencies have extensive capabilities for tracking and intercepting communications across Pakistan.
CIA contractor Raymond Davis, who was arrested in Lahore earlier this year after he gunned down two men believed to be linked to Pakistani intelligence agencies, was part of a cell that was tracking the LeT and its front organisation, the Jamaat-ud-Dawah.
Lakhvi is currently being held in Adiala Jail along with six other suspects charged with planning, financing and facilitating the 2008 Mumbai attacks that killed 166 people.
The trial of the seven Pakistani suspects by a Rawalpindi-based anti-terrorism court has virtually stalled for a variety of technical reasons.
Pakistani prosecutors have accused the lawyers defending the accused of filing a string of petitions in the Lahore High Court and other courts to delay or impede the proceedings in the anti-terrorism court.
Officials told PTI that the case against the seven Pakistani suspects was largely based on evidence provided by India and prosecutors and investigators had done little to collect material against the accused.
Lakhvi and several other LeT activists were arrested during an operation conducted by the Pakistan Army on December 7, 2009 at Shawai Nullah near Muzaffarabad, the capital of Pakistan-occupied Kashmir.
LeT founder Hafiz Mohammad Saeed and other JuD leaders were put under house arrest after the UN Security Council declared the JuD a front for the LeT.
However, Saeed was freed on the orders of the Lahore High Court about six months later.