Engineers found the problem at the bridge between Portsmouth Harbour and Portsmouth and Southsea stations today.
In a statement South West Trains said: 'During an inspection of the bridge between Portsmouth and Southsea and Portsmouth Harbour, an engineer identified a structural fault which must be rectified urgently.
'As a result, no trains are able to cross the bridge until the issue can be resolved.
'Trains may be cancelled, delayed by up to 20 minutes or revised.
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'Trains will have to terminate short at either Portsmouth and Southsea or Fratton, and they may not be able to run at all if there is not sufficient space in these stations.
'Other cancellations may be necessary, due to the fact that a number of trains were berthed in Portsmouth Harbour overnight, and are therefore now trapped in the station.'