I-level replaces the incumbent agency, Media Contacts, which has held the account since July 2002.
Dawn Whittaker, Sky's head of internet marketing, said: "Sky.com is a highly cost-effective channel for new customer acquisitions. We look forward to working with i-level to deliver continued progress towards our ambitious growth targets."
Charlie Dobres, the deputy chairman of i-level, added: "To win an account of the size and prestige of Sky is a fantastic testament to what I believe is an unrivalled quality of people, thinking and campaign results at i-level.
"Internet advertising in the UK has hit an amazing growth period so our approach since the beginning of the year has been to be constantly geared up to handle significant wins."
BSkyB added 95,000 new subscribers in the first quarter of 2005, taking its total to 7.7 million. It remains on course to hit its target of eight million by the end of the year.
The subscriber take-up has been boosted by Christmas sales and BSkyB's recent marketing blitz, driven by Sky's £20 million autumn marketing campaign "What do you want to watch?"
BSkyB's third-quarter revenues rose 10 per cent to £2.9 billion in the three months to 31 March. However, its churn rate rose to 11.1 per cent compared with 9.5 per cent in the previous quarter.
The i-level appointment is the latest in a series of changes to BSkyB's communications arrangements. It moved its media planning and buying from Universal McCann to MediaCom last year, and gave the brief to promote Sky One to Wieden & Kennedy at the start of this year.