The company says the impact of the Covid-19 pandemic has reduced global demand.
Triumph Motorcycles which is based in Hinckley says the proposed restructure is expected to result in 240 workers in the UK being laid off as part of 400 job losses around the world.
The firm said a consultation period would begin with employees.
In a statement the company said: "As a result of the ongoing impact of the Covid-19 pandemic, which has significantly reduced global demand for large capacity motorcycles, Triumph Motorcycles is to begin a consultation period with its employees in order to protect the long-term health of the business.
The proposed restructure is expected to result in circa 400 redundancies from across its global workforce, including approximately 240 from its UK headquarters and operations.
Sales in the larger 500cc plus motorcycle segment, in key markets such as France, Italy, Germany, the USA and the UK have fallen by between 40 and 65% over the past three months during what would normally be the peak season for sales.
Although Triumph sales have outperformed this significant decline to some degree, the market is forecast to remain considerably down on pre Covid 19 levels as a direct result of the economic conditions created by it."
Triumph Motorcycles Chief Executive, Nick Bloor, explained: "These are not only challenging times for everyone as individuals, but also for the company. No business could have anticipated the scale of the coronavirus crisis and its economic consequences.
"The pandemic has caused significant damage to the global motorcycle market, and, sadly, we have to respond and react accordingly as both a responsible employer and as a business that invests for the future.
"These are not easy decisions to make, especially when individuals' livelihoods are affected; however regrettably the scale of impact of Covid 19 necessitates us to restructure now in order to protect the long term health and success of the Triumph brand and business."