Relations between China and the United Kingdom are gaining momentum as British Prime Minister Theresa May is poised this week to begin the process of taking the UK out of the European Union.

"The China-UK relationship at age 45 is entering a new phase of increased maturity," said Chinese Ambassador Liu Xiaoming, as the two countries marked the 45th anniversary of ambassadorial relations on Monday.

Liu said the two countries must be "more innovative in our cooperation in broader areas and at greater depth to ensure that China-UK relations bear more golden fruit in this golden era".