Delays across the railways due to signalling failure

Update 11:05am: Network Rail says that the fault with their railway radio system in England and Wales has been traced to one of their major hubs near Stoke and a fix is underway. Most services should be back to normal by lunchtime.
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There is a serious fault affecting mainline railways and leading to delays and cancellations mainly across the southeast of England.Network Rail said that a fault with the communication system used between train drivers and signallers is causing the problems.