The most important feature of being a good and effective teacher is excellent subject knowledge, an expert has said.

Speaking at the Mayor's Education Conference at City Hall in London, Mitchell Chester, education commissioner for Massachusetts, US, explained that teachers who have a strong command over their subjects are some of the best professionals around.

He told delegates that this applies at all levels of the education system, from primary school onwards, TES reported.

"To become a teacher in Massachusetts you have to show strong subject knowledge," Mr Chester said.

"We feel that is an absolutely essential component. How can you be a teacher of mathematics or science if you are not strong in that subject?"

His own experience and that of Massachusetts speaks volumes. For example, the state is one of the highest performing in the US with regards to national tests.

Mr Chester told educators that this emphasis on acquiring strong subject mastery is about engaging in continuous professional development – to keep building on existing knowledge and skills.

His sentiments are shared by Pilvi Torsti, state secretary for the minister of education in Finland.

Ms Torsti explained that in-depth subject knowledge is one of the reasons why Finland has one of the best education systems in the world. It regularly achieves high scores in international rankings.

One of the requirements candidates looking to enter the profession need in the Nordic country is a master's degree (although this is not applicable for preschool teachers).

The Mayor's Education Conference is an annual event that is attended by UK-based headteachers and senior leaders the field of education.

They are able to meet, learn and discuss things with international experts, to "share great ideas that really work".