A source close to the party - whose aim is to see the UK withdraw from the European Union - said supporters had been encouraged to register as voting members of the website, without disclosing their party affinity.
By mobilising a large number of its members, many of whom are aged above 55, the UKIP tactic is an attempt to skew YouGov polls.
YouGov gives extra weight to older voters since they are less well represented among YouGov's membership base due to generally having less access to the internet.
YouGov director of policy research Stephan Shakespeare said the firm was aware of UKIP's campaign, and had taken measures to counteract it: 'We've caught them all out. We monitor new members all the time and any unusual changes to recruitment patterns are addressed.'
Shakespeare added that the firm was unperturbed by allegations from UKIP that a YouGov poll for The Daily Telegraph was biased against the party, stating: 'We ask the questions our clients request us to.'
The UKIP claims that a YouGov poll on voting intentions of 18 to 22-year-olds, published in The Daily Telegraph on 2 July, excluded the UKIP from the polling, despite the inclusion of smaller parties such as the Green Party and the BNP.
In an e-mail to the UKIP's campaigners, YouGov said The Daily Telegraph had specifically requested information on the Greens and the BNP among this age group, leading to the UKIP's omission from the poll.
But UKIP press officer Mark Croucher said he thought the move was politically motivated: 'We have great difficulty gaining media coverage, because of the political agenda of the press.'
Croucher added that his party believes the 2 July survey breached market research guidelines: 'Had YouGov belonged to one of the trade organisations they would have had to abide by these. It's hardly impartial to ignore the UK's fourth largest political party.'
But Shakespeare defended YouGov's policies, reiterating the absence of guidelines to cover online polling: 'All of the guidelines that can reasonably be applied to online polling, we do apply.'