Haneef, also an Indian doctor, has been in custody in Australia following his arrest on July 2 in the eastern city of Brisbane as he was attempting to leave the country on a one-way ticket.
Ahmed was arrested in Liverpool, north-west England, on June 30, after a burning Jeep crashed into a Glasgow airport terminal.
He worked as a doctor at Halton Hospital in Runcorn, Cheshire, also in north-west England.
Scotland Yard said Ahmed is accused of having information that could have helped police prevent another person from committing a similar attack, or enable police to arrest and prosecute them.
Police initially detained eight people on suspicion of involvement in the attempted bombings - seven in Britain and one in Australia.
Three people arrested in the immediate aftermath of the attacks remain in custody. They are Jordanian doctor Mohammed Asha, 26, and two trainee doctors aged 25 and 28, whose identities have not yet emerged.
Under British law police can detain and question them for up to 28 days, subject to regular judicial review.
Scotland Yard will have to apply to a magistrate this weekend if they want to hold them further.