Figures from Oil and Gas UK (OGUK) say the average weekly workforce dropped from 11,000 to 7,000, during March.
A halt in drilling and engineering construction are being blamed for the sudden loss of 4,000 jobs.
Although numbers offshore have since risen steadily they only peaked at 9,000 in August.
OGUK said it would be next year before it could fully quantify the impact of Covid-19.
However initial indications are described as "worrying".
The industry body has called on government, regulators and the industry to work together to protect jobs, which it says could be valuable for a transition to a lower carbon economy.
The Workforce Report 2020 says uptake of the Coronavirus Job Retention Scheme appears to be high, particularly in the oil and gas supply chain.
Low oil and gas prices on top of the lockdown added to the industry's woes.
OGUK workforce engagement and skills manager Dr Alix Thom, said: "Our figures confirm the initial operational impact of the lockdown back in March this year, with the number of workers offshore decreasing considerably in the space of a month as companies reduced to minimum manning in a bid to control the spread.
"We continue to see some very worrying signs for employment in the sector, with the uptake of furlough and continued suppression of global energy demand impacting our industry like many others in the wider economy."
She added: "The recruitment and retention of diverse and talented people will be essential as we work to support UK energy needs both now, and in a lower carbon context."
Read the full story here.