Most of Mitt Romney's newest Twitter followers are fake, according to an investigation of bogus social media accounts.

A pay-for-follower service most likely drove the presumptive Republican nominee's recent and dramatic spike in online followers, concluded Baccardua Labs, a digital security company.

The widely reported surge in tens of thousands of new followers for @mittromney from 21 July – which provoked commentary and suspicion – appeared to have been purchased from a dealer, it said: "We believe most of these recent followers of Romney are not from a general Twitter population but most likely from a paid Twitter follower service."

The analysis, part of a wider investigation into what the report called the underground Twitter economy, found telltale signals that about a quarter of the new followers were less than three weeks old and had not tweeted. Some 80% were less than three-months-old.

The report's author, Jason Ding, said there was no way of identifying whether it was the work of the Romney campaign, a Romney supporter or an opponent out to discredit him.

"Romney's newest followers could have been paid for by himself, his associates or by his opponents. So far, there is not a feasible way to confirm who is responsible."

Authentication was not required when buying Twitter followers from eBay or other websites, he said, and anyone could buy followers for other Twitter users.

As of Wednesday, Romney had nearly 800,000 followers.

Zac Moffat, the Romney campaign digital director, denied that his side was responsible. "The Romney campaign does not buy Twitter followers," he told CNET. "We have reached out to Twitter to find out additional information regarding the rapid growth."

By Wednesday afternoon @mittromney had more than 792,800 followers. He gained 116,922 in a single day, 21 July. Around a tenth have since been suspended by Twitter.

Barracuda said this fit a wider pattern of clandestine Twitter trading which it began studying in May. "Our team set up three Twitter accounts and purchased between 20,000 and 70,000 Twitter followers for each of them from eBay and another website searched from Google."

It identified "dealers" who charge an average of $18 for 1,000 followers. A dealer can earn up to $800 a day for 7 weeks of selling followings if they can control 20,000 fake accounts, it said. They can earn extra revenue by selling tweets and re-tweets.

The report defined dealers' clients as "abusers", with the average abuser boasting 48,885 followers. The phenomenon of fake accounts is not new. The oldest was said to be @krails, created on 15 January 2007. Dealers controlled the following speed and total following number of fake accounts to avoid being suspended by Twitter. "Dealers can apply obscure techniques to make them hard to detect, eg randomly following some famous and some average people, or posting tweets grabbed from the Twitter stream, etc."

Prices for 1,000 followers ranged widely from $2 to $55 depending on how "real" they seemed. "This underground Twitter business is just blooming," the report said.