Metropolitan Police chief Sir Ian Blair has criticised his City of London colleagues for shutting down the mobile phone network on 7 July.
The 7 July London bombers killed 52 commuters
As suicide bombers attacked the city's transport network, many Londoners found their phones had lost their signal.
The network, within a mile of Aldgate Tube station, was shut for four hours.
"I don't think it was appropriate, I don't think it helped and as soon it was found out that it had happened it was reversed," Sir Ian said.
He and mayor Ken Livingstone were appearing before a review committee at City Hall, which is looking into the response to the attacks.
However, the City of London force said it closed the network after problems with its own phones.
A spokesman said: "On July 7 City of London Police encountered difficulty with mobile phone communication across its mobile phone provider's network (O2).
"To ensure the best operational response for the City of London community and to facilitate effective communication, the City of London Police requested O2 to implement the Access Overload Control Scheme.
"It was a rapidly moving situation and when it was established that this was no longer required City of London Police made a request to O2 to reinstate normal working practices."
Mr Livingstone said the fact that he had been in Singapore at the time, celebrating London winning the 2012 Olympic bid had not made a major difference to his role.
He told the committee that communications were so good that he could speak to the same people he would have if he had been at City Hall.
But he warned that Londoners could expect more attacks in the future.
"We know there have been attempts since July. This is not something we have got through and is over and behind us. We are midway between that attack and the next one," he said.
He praised the emergency services for their response to the attack and said, next time, capabilities would be even better.
"We will be able to handle the next wave of attacks better than this," he said.
"We were stretched to the limit on this. I suspect the terrorists planning their next attacks will aim to have more incidents."