LONDON (Reuters) - Britain on Thursday said China had broken its main bilateral treaty on Hong Kong by imposing new rules to disqualify elected legislators in the former British colony, cautioning that it would consider sanctions as part of its response.The British flag was lowered over Hong Kong when the colony was handed back to China in 1997 after more than 150 years of British rule - imposed after Britain defeated China in the First Opium War. Hong Kong’s autonomy was guaranteed under the “one country, two systems” agreement enshrined in the 1984 Sino-British Joint Declaration signed by then Chinese Premier Zhao Ziyang and British Prime Minister Margaret Thatcher. “Beijing’s imposition of new rules to disqualify elected legislators in Hong Kong constitutes a clear breach of the legally binding Sino-British Joint Declaration,” Foreign Secretary Dominic Raab said. “China has once again broken its promises and undermined Hong Kong’s high degree of autonomy.” Britain summoned China’s ambassador, Liu Xiaoming, to express its deep concerns and Raab’s deputy, Nigel Adams, told parliament that it was considering possible sanctions on individuals over China’s actions. “We will continue to consider designations under our Magnitsky-style sanctions regime,” said Adams, Britain’s minister for Asia, referring to sanctions similar to those imposed on those deemed responsible for human rights abuses under the U.S. Magnitski Act. He was asked by lawmakers if Britain would sanction Hong Kong Chief Executive Carrie Lam.