“We have reached the conclusion of our partnership, the contract expired on 31 December by mutual agreement,” a team spokeswoman said on Wednesday.
The AT&T branding had also been removed from the team website and list of partners.
Media reports indicated the AT&T deal had been worth around $7 million a season, a comparatively small sum compared to title sponsorships at many other teams but still an important contribution to the overall budget.
The British-based team endured their worst season in Formula One last year, taking just five points from 19 races and finishing ninth overall.
The former world champions lost two blue chip sponsors at the end of 2010, with Dutch electronics company Philips and troubled bank and previous title backer Royal Bank of Scotland (RBS) departing.
However the team, who listed in Frankfurt last March, gained Venezuelan state oil company PDVSA along with driver Pastor Maldonado.
They have also built up a relationship with Qatar, where they have a technology centre, and are hoping to strengthen those ties further with reports doing the rounds that a deal involving a consortium of Qatari companies – involving local telecoms company Qtel and QNB (Qatar National Bank) – is on the cards.
Williams have a high profile presence in the Middle East country with their partnership with the Qatar Science & Technology Park (QSTP)in the Williams Technology Centre (WTC) which is a world class incubator for the research, development and commercialisation of new technologies.
A team source said the AT&T deal had not been ‘sustainable’ at the existing level and Williams were now in advanced negotiations with another telecommunications company for the title sponsorship. No details were given.
Williams have yet to decide their second driver for the 2012 season starting with the Australian Grand Prix in Melbourne on March 18, and word is any decision will be dependent on the title sponsorship package.
Brazilian Rubens Barrichello is hoping to be kept on for his 20th season in F1 but faces strong competition from Adrian Sutil.
Russian Vitaly Petrov and Brazilian Bruno Senna, who may convince Embratel to take over the title sponsorship, are among several other 2011 drivers also looking for seats after being dropped by Renault, who will race this year as Lotus.