Boris Johnson was under fire on the first day of the formal election period after his re-election team appropriated the official City Hall Twitter account, by switching the name @MayorofLondon to "@Boris Johnson", transferring in a stroke the 253,144 twitter followers tracking the activities of the London mayoralty to Johnson's re-election campaign.

To the fury of the Labour party, Johnson's campaign team also replaced the website link directing readers to the City Hall website, www.london.gov.uk, with the link to his re-election campaign site, backboris2012.com.

Labour believes that the decision to migrate the official mayoral Twitter account, set up in May 2008 and managed by the publicly funded Greater London authority, to support Johnson's re-election bid represents a misuse of resources and has written to the Greater London authority's monitoring officer, copied in to the district auditor, asking him to investigate.

Johnson has previously been warned about using the mayor of London Twitter account for party political purposes.

GLA rules stipulate that elected members "must not use resources improperly for political purposes [including party political]".

The mayor's press office confirmed that the account was set up after Johnson was elected mayor in 2008.

A member of Ken Livingstone's team said: "Basically taxpayers money has been spent to build up and maintain a feed that Boris has now hijacked. To compare like with like, the No 10 Twitter feed was set up under Gordon Brown but has stayed as an official feed since [David] Cameron took over."

But Johnson's campaign stood by its decision to switch the name, claiming that the account was owned by Twitter, not the GLA. A spokesman added that it was Johnson's brand that had attracted the followers to the account in the first place and that there had been a "surge" in followers following the transfer of name – a claim disputed by Labour, which insists the account had 253,125 followers before the switch.

A spokesman for Johnson's re-election campaign said: "Boris has been completely transparent about changing the Twitter name and since it has changed we have seen a surge in followers despite followers being free to unfollow at any time."

In a swipe at Livingstone, Johnson's Labour rival, he said: "I don't think anyone is any doubt that the amount of followers have been built up on this account are directly because it is Boris Johnson's. Does anyone really think that a man who has just 5,000 Facebook friends compared to Boris's 125,900 would have built up a following on Twitter of 253,100? The simple fact is that all Twitter accounts are owned by Twitter and this one was set up at zero cost to the taxpayer."

In 2009, a complaint was lodged after Johnson used the official Twitter account to celebrate the fact that the Sun newspaper had decided to back the Conservative party at the 2010 general election. He tweeted: "The Sun has got his hat on, hip hip hip hip hooray" and linked to the Sun website in September.

The GLA panel dealing with complaints concluded that the mayor "could have been seen" to have been breaching a clause in the code of conduct governing elected members, "as it appeared on the evidence presented that the mayor of London was using GLA resources in seeking to affect party political support".

Len Duvall, a Labour party London assembly member who is seeking re-election in May, believes that the rules may have been breached again.

He wrote to the monitoring office: "My understanding is that this Twitter account was established by the authority and belongs to the office of the mayor of London, and has been maintained since its inception by GLA resources. As such, this account does not belong to the current mayor of London, Boris Johnson. Can you quickly establish the facts and intervene on this abuse and misuse of resources in undertaking this transfer?"

He added: "If it is found this is a personal account, it has featured on a number of GLA and mayoral publications and I would expect the organisation to be reimbursed the full sum that have been spent on this personal and political campaigning."