A whistleblower has warned that a system of safety representatives set up to underpin the integrity of oil and gas platforms in the North Sea following the Piper Alpha explosion is being made a "mockery" owing to an atmosphere of offshore bullying by some managers.

The claims come just before tomorrow's formal 25th anniversary of the rig disaster – the world's deadliest offshore accident in in which 167 workers died.

The whistleblower, who has spent 20 years in the industry, claims some serious injuries continue to be covered up rather than reported while "lost time accidents" – a key industry safety measurement – can be manipulated by keeping injured employees on board a rig until it is time for their leave.

"Under Health and Safety Executive regulations a company is duty bound to look after workers by way of safety reps and their regular meetings. But it's a mockery. They don't often happen and top line supervisors risk losing bonuses if there are too many lost time incidents, so (injured) people are kept on board so they don't have to report it," said the offshore worker, who asked not to be named.

The whistleblower says there are many North Sea safety reps who never attend any safety sessions despite the HSE regulations calling for each group of workers to hold discussions every three months.

The offshore worker, whose identity is verified by the RMT union in Aberdeen, also said one of his colleagues had been re-elected as a safety rep by the workforce but his appointment had been overruled by the employer.

Jake Molloy, an Aberdeen-based organiser of the RMT, supports the claims made by the whistleblower, and estimates that the safety rep structure and industrial relations, in general, are either "poor or completely dysfunctional" on 40% of North Sea rigs or platforms.

He believes lost time injury statistics are "meaningless" and said BP's Deepwater Horizon accident in the Gulf of Mexico in 2010 showed the dangers of what happens when workers feel unable to challenge and halt drilling in dangerous situations.

"You can have the most technologically advanced rig on the planet like the Deepwater Horizon but it's now lying on the bottom of the sea with 11 dead … the danger of another Macondo is always real, especially on high-pressure wells in deep water off the (UK) west coast," he said.

Last month the HSE said the number of large gas and oil leaks from the UK's offshore installations rose in 2012 from three to nine, the highest in 14 years. Included in those figures was a leak that went on for almost two months on the Elgin platform operated by Total of France.

In late 2010 the same safety watchdog sent a report to rig operator, Transocean, in which staff alleged they had been bullied and harassed, something that was denied by the US company.

And in 2007 the HSE issued a report following a study showing that almost 60% of North Sea oil platforms had rusting or other problems that oil companies should have addressed.

But Oil & Gas UK, the employers' group, issued its latest annual health and safety report two week ago in which it reported a 48% reduction in the number of reportable oil and gas releases over the last three years and an all-time low last year in the incidence of "over-three day injuries", which it regards as key measures for good industry performance.

Sally Hatch, an Oil & Gas UK spokeswoman, said on Thursday eveningthat it did not recognise the picture painted by the whistleblower of bullying and intimidation.

"We are not aware of that scenario. Industry representatives are represented at the highest level with the RMT sitting on the Step Change in Safety partnership. Safety reps are trained by the oil and gas industry and are given all the tools and knowledge needed to represent their colleagues," she said.

"Reporting of offshore lost time injuries is very high. HSE inspectors periodically check first aid treatment records kept on the installation by the offshore medic as part of their planned inspection activities."

But she confirmed that only 91 of the 2,000 safety reps working across the North Sea had appeared at the Workforce Involvement Day at the Piper Alpha 25 event, which ran for three days and was a key event for discussing industry safety.

Frank Doran, whose first stint as an MP for Aberdeen coincided with the accident, said he believed there had been "huge improvements" in the general safety culture in the North Sea but he also warned there were still gaps in the system.

"There have been some pretty shocking figures on gas releases and some of contracting companies come under pressure from the big boys (larger groups) but the general trajectory is in the right direction. At the UK (oil and gas industry) safety awards the main speech was given by the general secretary of the Scottish TUC. That would have been unthinkable in the past."