Green leader Natalie Bennett, Ukip's Nigel Farage and SNP chief Nicola Sturgeon are the most engaged political leaders on Twitter, according to figures.

With social media set to play an unprecedented role in the general election campaign, the parties are all trying to work out how best to deploy it.

But a survey of their tweeting habits shows the leaders have been taking markedly different approaches.

David Cameron largely uses his @David_Cameron account to make statements and tell voters what he is doing. The Tory leader has the largest follower base, at nearly 890,000 - up by 3.4% over the past month.

However, the Prime Minister only pumps out an average of two tweets a day - and only 2% of those are in reply to another user.

Ed Miliband, meanwhile, has nearly 370,000 followers - a rise of 1.7% since December. He also generally tweets twice a day.

The Labour leader is far more likely to engage though, with 12% of his messages being responses to other people.

Lib Dem leader Nick Clegg tweets to his 202,000 followers just once a day on average, with 7% of those replies to users.

Both Mr Cameron and Mr Clegg also have official accounts as Prime Minister and Deputy Prime Minister, which are operated by civil servants and used for official business.

The leaders of the traditionally smaller parties are notably more likely to get involved with the public, according to the data.

Mr Farage - whose followers have increased by 3.6% over the past month to 179,000 - tweets four times a day and 24% of those are replies.

Scottish First Minister Ms Sturgeon is even more prolific at eight tweets a day on average, with the same proportion being replies. Her followers have risen 7.5% to just below 116,000.

But Green Party leader Natalie Bennett manages 23 tweets per day, 31% of which are replies. Since December there has been a 9.8% increase in her followers - albeit from a lower base - to 40,661.

Sean Kemp, a former special adviser to Mr Clegg in government and now a public affairs consultant, said as leaders become more prominent it becomes harder for them to converse on Twitter.

Apart from being too busy to maintain the account themselves, they also receive too much abuse.

"The amount of abuse you get, it is impossible to engage. To get to the person who wants a conversation about the NHS, you have got to get past 50 people who are saying 'go away and die'.

"That means major leaders' accounts end up being quite safe, boring accounts."

Mr Kemp said Mr Farage is a good example of maintaining engagement with relatively large follower numbers.

"It is a bit first person and relatively regular," he added.

Mr Kemp said parties would be using social media extensively in the campaign - not least because it is "cheap".

But Twitter would be used more to row with each other, and get their messages to journalists and existing supporters, rather than for trying to attract new people into the fold.

"Twitter is not massively useful for that, because you are speaking to people who are already engaged. On Twitter you tend to get people shouting their opinions at each other.

"Parties tend to use Facebook as a better shot for political engagement. You can get people to share more, people can take action, sign up for things.

"You just need one person putting information out there as a Lib Dem, and they are reaching lots of ordinary people."