Image copyright Reuters Image caption Google's own workers have raised concerns after reports it is developing a censored search engine for China

A former Google employee has warned of the firm's "disturbing" plans in China, in a letter to US lawmakers.

Jack Poulson, who had been a senior researcher at the company until resigning in August, wrote that he was fearful of Google's ambitions.

His letter alleges Google's work on a Chinese product - codenamed Dragonfly - would aid Beijing's efforts to censor and monitor its citizens online.

Google has said its work in China to date has been "exploratory".

Ben Gomes, Google's head of search, told the BBC earlier this week: "Right now all we've done is some exploration, but since we don't have any plans to launch something there's nothing much I can say about it."

A report by news site The Intercept last week alleged Google had demanded employees delete an internal memo that discussed the plans.

Google has not commented on the staff row, but said: "We've been investing for many years to help Chinese users, from developing Android, through mobile apps such as Google Translate and Files Go, and our developer tools."

It added: "We are not close to launching a search product in China."

'Censorship blacklist'

Mr Poulson's letter details several aspects of Google's work that had been reported in the press but never officially confirmed by the company. It will be submitted to the Senate Commerce Committee, which is holding a hearing on Wednesday in Washington DC.

The topic of the hearing is "examining safeguards for consumer data privacy".

Google's chief privacy officer, Keith Enright, is due to appear - as are representatives from AT&T, Apple, Twitter and Amazon.

The letter alleges Google is working on:

A prototype interface designed to allow a Chinese joint venture company to search for a given user's search queries based on their phone number

An extensive censorship blacklist developed in accordance with Chinese government demands. Among others, it contained the English term "human rights", the Mandarin terms for 'student protest' and 'Nobel prize', and very large numbers of phrases involving 'Xi Jinping' and other members of the CCP

Explicit code to ensure only Chinese government-approved air quality data would be returned in response to Chinese users' search

Mr Poulson said the sum of these efforts amounted to a "catastrophic failure" of Google's internal policies on privacy - as well as going against assurances made to the US trade regulator regarding data protection measures in its products.

"Dragonfly is part of a broad pattern of unaccountable decision making across the tech industry," Mr Poulson wrote.

Mr Poulson's letter follows a joint statement signed by hundreds of current Google employees against Dragonfly sent last month.