Motorcyclists caught by speed cameras could escape fines and points due to the postal strike.
The High Court has quashed a conviction because police failed to notify the driver within the 14-day legal time limit from the date of the alleged offence.
Thousands more speeding notices could be delayed as backlogs are cleared, raising the prospect of many more motorists using the defence.
Lord Justice Elias said that during the delays: “The authorities must then adopt other means of warning, if they are to avoid the risk of late delivery.”
The notices are sent by normal post despite ACPO guidelines saying recorded delivery should be used.
A spokesman for ACPO Road Safety Support said no new guidance had been issued. “We’re reviewing it,” he added.
Speed camera campaigner Captain Gatso said: “They will have to get community support officers to hand deliver them.”
Road traffic lawyer Robert Dobson advised riders who received notices late to keep the envelope and note on it the arrival date.