major concern

You usually use it as a noun phrase after the verb is or has become. You can say 'The weather is a major concern for our picnic.' It works perfectly when you want to sound serious but calm. It is like pointing at a crack in a dam; you aren't screaming, but you are definitely worried. Use it to highlight the most important obstacle in your way.

Use it in meetings when a project is over budget. Use it with your doctor when describing a symptom that won't go away. You can even use it with friends when you are worried about someone's choices. It is great for news reports, school essays, or professional emails. It sounds smart and balanced. It shows you have thought about the situation deeply.

Don't use it for tiny, silly things unless you are being funny. If you run out of milk, calling it a major concern sounds a bit dramatic. Avoid it in very high-energy, emotional arguments where you might use stronger words like 'disaster.' Also, don't use it if the problem is actually quite small. You don't want to be the person who cries wolf over every little thing.

In English-speaking cultures, especially in business, people often avoid being too blunt or aggressive. Instead of saying 'This is a terrible mistake,' they say 'This is a major concern.' it is a polite way to be firm. It allows everyone to stay professional while acknowledging that something is going wrong. It is the 'polite warning' of the corporate world.

You might hear people say primary concern or chief concern. These mean almost the same thing but sound slightly more formal. If the problem is growing, you might hear growing concern. If it is a shared worry, it becomes a matter of concern. All of these help you adjust exactly how much 'worry' you want to express.