LONDON (Reuters) -Britain on Tuesday summoned the chargé d'affaires of the Chinese Embassy in London after accusing Chinese state-backed hackers of stealing data from Britain's elections watchdog and carrying out a surveillance operation against parliamentarians.

"The (Foreign Office) set out the government's unequivocal condemnation of Chinese state-affiliated organisations and individuals undertaking malicious cyber activity against UK democratic institutions and parliamentarians," a spokesperson for Britain's Foreign Office said in a statement.

A spokesman for Prime Minister Rishi Sunak said on Tuesday the government is close to finalising a new foreign influence registration system that would require anyone working undeclared for a foreign country in the so-called "enhanced tier" to declare their activity.

Under Britain's new National Security Act, individuals, such as lawyers, a public relations company or an undercover spy working for a country in the "enhanced tier" would have to record their activity in a register or face prosecution.

British Deputy Prime Minister Oliver Dowden said on Monday that China's alleged hacking of British democratic institutions meant there was a "strong case" for including the country in the enhanced tier.

China has denied the spying allegations. The Chinese embassy in London said on Monday the claims said the claims were "completely fabricated" and it will make "a justified and necessary response".

The British government has previously said it would be inappropriate to call China a "threat" because it is too simplistic to view relations with the world's second biggest economy through a single word.

(Reporting by Sarah Young, Andrew MacAskill and Sachin Ravikumar; Editing by Alistair Smout)