Mobile phone users in the London areas affected by today's bombings are reporting difficulties in getting access to mobile networks.
Vodafone said in a statement: "Our London switches are at capacity (which is very rare) so we are having to go into 'access overload' procedures which means freeing up a proportion of the capacity across London to ensure that the police and emergency services can communicate.
"Therefore customers will experience temporary issues making calls but we urge them to keep trying."
A spokesman for O2 said: "There is no damage to our network but there is congestion in some areas which we are working to overcome.
"We have not been told by the government to place restrictions on usage, although that is something that can be done."
A T-Mobile spokesman said: "There is no physical damage to the network but we are experiencing congestion. Subscribers are experiencing problems and it will take time to resolve."
A spokesman from Orange added: "We have offered to prioritise services but have been asked not to do so. Some customers are experiencing problems, and we urge them to keep trying."
Problems appear to have spread beyond central London. A mobile phone user in Bristol said that it had taken him 50 minutes to contact his brother in Richmond who was using the T-Mobile network.
Others reported problems in Blackheath and Hemel Hempstead.
The friends and family hotline for the London explosions is 0870 1566 344.
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