Thousands of speeding motorists may be able to escape fines and penalty points on their licences thanks to the postal strike.

In what is being seen as a test case, a High Court judge has quashed the conviction of a driver because the statutory police letter failed to arrive within the 14-day legal deadline. It had been delayed by Royal Mail industrial action.

The ruling by Lord Justice Elias could lead to a flood of refusals to pay fines by thousands of drivers whose letters were caught in the backlog caused by the strike. It could also force police forces to abandon the Royal Mail to deliver the notices.

The case opens up the prospect of motorists who have received a police letter claiming that they did not do so within the 14-day limit and thereby escaping prosecution. The letters are sent by normal post.

The precedent has been set by the case of Peter Gidden, 48, who was caught by a speed camera driving at 85mph on a motorway during the last big postal strike in October 2007. The statutory notice of intended prosecution was caught in a backlog of undelivered mail and did not arrive until 16 days later. Elias ruled last Thursday that the prosection should not stand, wiping out three points imposed on Gidden’s licence and £680 in fines and legal costs.

“This case raises an issue of some topicality given the current postal strike and is of no mere small interest,” Elias said.

“I appreciate that this construction of the legislation may create problems for the police and prosecuting authorities, particularly when the postal service is on strike with the inevitable delays in delivery.

“The authorities must then adopt other means of warning, if they are to avoid the risk of late delivery.”

According to official figures, 1.46m speeding tickets were issued in England and Wales during 2007, the last year for which figures are available. The typical penalty is £60, suggesting the fines raised at least £87m that year. The backlog of letters in this year’s strikes has reached 50m, suggesting thousands of letters could be delayed beyond 14 days.

Captain Gatso, campaigns director of Motorists Against Detection, who works using only his pseudonym, said: “Police and councils will be furious with this decision. It leaves the onus on the authorities to ensure [notices] get there in 14 days — motorists can simply argue it didn’t arrive on time. This is a volcano that could destroy the entire system.”

The backlog of letters is expected to grow again after the Commercial Workers’ Union last Friday announced two days of strikes this Friday and on November 9. They are expected to be on a far larger scale than the action so far.

Chris Phillips, an analyst at Post-Switch, a firm that helps businesses sign up with Royal Mail’s rivals, said: “This is a live and serious issue for the police.

“We have already helped one police authority in the north which sends out 40,000 penalty notices a year sign up with one of Royal Mail’s competitors. We have had interest from other police forces in recent days.”

There are signs this weekend that the strike is becoming increasingly acrimonious, with claims by the union that dozens of postmen are being disciplined and even sacked after posting abusive messages about Royal Mail management on Facebook, Twitter and other social networking websites.

Simon Walker, from the CWU’s Derby branch, said he had seen seven members reprimanded over postings on social networking websites since the strikes began. A Facebook group called “Time for Adam Crozier to go” has attracted more than 1,250 members and displays an unflattering photograph of Royal Mail’s chief executive in his swimming costume.

“He couldn’t run 100 metres let alone Royal Mail,” one of the less offensive comments reads. Another member writes: “Adam Crozier is nothing more than a bully boy leech ... the sooner he leaves the better.”

According to local CWU officials, the Royal Mail’s investigations branch have been trawling the website. The union is now advising its members to use pseudonyms when posting critical messages.

A spokeswoman for Royal Mail said it was “nonsense” to suggest the company disciplined staff for criticising management, but added: “We do always take any allegation of bullying or harassment very seriously and would clearly investigate." Talks between the CWU and Royal Mail are expected to continue this week.

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