China has passed a controversial security law giving it new powers over Hong Kong, deepening fears for the city's freedoms, the BBC has learned.

Last month China announced it would impose the law, which criminalises any act of secession, subversion, terrorism or collusion with foreign forces.

The move comes after angry protests last year - sparked by another law - which became a pro-democracy movement.

Critics fear the new law could be a greater threat to Hong Kong's identity.

They warn it will undermine Hong Kong's judicial independence and destroy the city's freedoms, which are not available in mainland China.

Hong Kong was handed back to China from British control in 1997, but with a unique agreement which guaranteed certain freedoms.

The bill has sparked demonstrations in Hong Kong and drawn international condemnation since it was announced by Beijing in May.

But China says the law is needed to tackle separatist activity, subversion, terrorism and collusion with foreign elements - and rejects criticism as interference in its affairs.

The security law was fast-tracked to come into effect before Wednesday, which marks the anniversary of the handover from Britain to China and is usually marked by large-scale political protests.

It was passed unanimously on Tuesday morning by the Standing Committee of the National People's Congress in Beijing and is expected to be added to Hong Kong's Basic Law later in the day.

China has not officially confirmed the law has been passed, and the text of the bill has also not been made public.