After joining the social networking service in March, “SarahBrown10” has won the attention of more than 774,000 followers, outstripping the 768,000 following the comedian, who was long the UK’s most high-profile “tweeter”.

Mrs Brown has a long way to go before overhauling the biggest Twitter stars, like singer Lily Allen, who broadcasts her thoughts to more than 1.5 million via her feed at “lilyroseallen”.

But the army of fans listening into Sarah’s tweets amounts to almost five times the entire membership of the Labour Party.

Mrs Brown steers well clear of political controversy in her messages, granting followers glimpses into the day-to-day life of a Prime Minister’s spouse and publicising her favourite charities rather than promoting Labour policies.

Yesterday, she tweeted on the G20 dinner in Pittsburgh in the “lovely environment” of an organic farm, as well as urging followers to sign up to the “Million Mums” campaign against mortality in childbirth.

Since becoming a Tweeter six months ago, Mrs Brown has sent out 1,162 messages, which are each limited to a total of 140 characters.

Online public relations expert Ross Furlong said that Sarah’s tweets may help Labour, even though she never uses them for campaigning purposes.

“Although the content is deliberately not party political, she is effectively pressing voter flesh online, as she did in person at the Glenrothes by-election to great effect,” said Mr Furlong.

Politicians who have dipped their toes into the world of micro-blogging are trailing far behind Mrs Brown in the popularity stakes. London Mayor Boris Johnson has about 48,000 followers, while the official Labour Party twitterfeed has about 6,000 and the party’s “Twitter tsar” Bristol East MP Kerry McCarthy 2,700.