Many people will have come across the phrases “best endeavours” and “reasonable endeavours” in their business lives and will have their own views as to what each means. If you have never heard of the phrases before, “reasonable endeavours” essentially means taking one, but not necessarily all, reasonable courses of action. “Best endeavours” is a higher obligation, which means doing all you reasonably can, even potentially acting against your own commercial interests. But take great care before inserting or accepting terms such as these into contracts you may sign or letters you may send. It can have unintended consequences.

Often a contract can require you to do (or not to do) something. But if the contract actually requires you to achieve an outcome, then this unqualified obligation could meet an open-ended nightmare if you have agreed to use your “best endeavours” to meet it.

The Court of Appeal forced an airport operator to accept arrival and departure flights outside the airport's usual opening hours because they had agreed to use "best endeavours" to promote an airline's low-cost service, even though none of this was mentioned in the contract and the operator would incur losses by doing so. A heavy burden indeed.

 And we haven’t even mentioned “reasonable endeavours” or, worse still, “all reasonable but commercially prudent endeavours” – messy hybrids than even the Courts are struggling to interpret. Be careful that you understand what you are agreeing to.