The Telegraph Media Group is conducting an internal investigation after a stray blog posting on Telegraph.co.uk sparked a Metropolitan Police investigation into electoral irregularities in London's Ealing and Southall by-election.

"There will be measures taken to refresh knowledge of electoral law across the organisation," a Telegraph source told Journalism.co.uk.

"There is a recognition that there are areas of media law that go beyond the usual - libel, contempt, and other issues."

The incident illustrates the pitfalls news websites face as they look to ramp up their online coverage, and the need to tread carefully when reporting on elections.

An article on the Little and Large political blog claimed to reveal the results of the postal ballot in the fiercely contested local election, based on information from "a source inside the Tory campaign".

The posting appeared on the Telegraph site on Wednesday July 18, ahead of the by-election the following day, and was removed after only a few hours following an external complaint that it might be in breach of voting rules.

Revealing the results of a ballot before polls have closed could be deemed an offence under the Representation of the People Act 1983, which governs electoral conduct in Britain.

The Conservatives denied to the BBC that anyone from their camp in Ealing and Southall spoke to the newspaper, and the Metropolitan Police says the allegation is being "thoroughly investigated".

Telegraph correspondents' blogs, mixing personal and expert views, have been an important addition to the site, which first launched in 1994, and a style guide was introduced last year to give advice on subject matter for postings - although it does not touch on legal issues.

It's not only newspaper websites that risk falling foul of electoral law. The Electoral Commission, which oversees the democratic process in the UK, is increasingly uncomfortable about a lack of regulation governing election campaigning online.

At the moment there is a legal requirement to include an imprint on all printed political material, stating whom it promotes and who produced it.

Although encouraged as good practice, there is no similar requirement for blogs and websites, muddying the waters of what constitutes political comment and outright party political promotion.

The Commission has passed its concerns up the line to its newly formed parent, the Ministry of Justice.

"It's definitely becoming more of an issue, and it's something we are constantly reviewing," said a spokesperson.

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