Do I need a visa to travel to Mexico through the US?

I am a UK citizen, who normally would qualify under the visa waiver program. However, I have had confliciting information on my need for a visa when transiting through the US.

I have read that if going to Mexico via US, then I DO need a visa, and the visa waiver does not apply. So I phoned the US Embassy in London who said it was OK, and I don't need a visa. But the US Embassy webiste says to travel to Mexico if I'm not a Mexican citizen, then I do.

I already have the visa waiver, as I travel to UK - US - UK, but I've never traveled UK - US - Mexico, and I don't want to be caught out at US Immigration!

Has anyone done this trip and who actually knows???

Thanks

PS. I have the up to date passport, no criminal record etc. I'm good to go.
I mean a visa for the US, not for Mexico. I hold an FM3 work permit for Mexico. It's whether I need a visa for transit in the US that I'm asking!!

Your info is a bit confusing to me. The Visa Waiver Program in the U.S. has nothing to do with Mexico. What you do need for Mexico is an FMT...commonly called the tourist card. It is actually a type of visa, but not in the sense that you apply for it ahead of time and get it stamped in your passport. You just pay the fee (about $26.00usd) and get the card at the border or on the plane. ( The fee is included in your plane fare if you are flying to Mexico.) When you are in the U.S. on the VWP, you are allowed to visit Mexico and return to the U.S. with your I-94 with no problem. In other words, taking a side trip to Mexico will not invalidate or "mess-up" your being in the U.S. under the VWP. See the site below and read the paragraph just ABOVE the sentence in brown print that says,"When does a national of a VWP country ...."

you don't need a visa if you're going on a connecting flight through the airport.

If you transit through the US to Mexico you will just have to apply for an ESTA (travel approval) on the website of the U.S. Department of Homeland Security for transiting under the visa waiver program since January 2009. .. http://www.cbp.gov/xp/cgov/travel/id_visa/esta/ ..http://www.cbp.gov/xp/cgov/travel/id_visa/esta/esta_question/02062009.xml ..It asks for the same information than on the green visa-waiver form in the airplane to include your phone-number and e-mail-address and is free of charge. If you plan to transit through the U.S. en route to Mexico you just need to omit the address information and fill "in transit to Mexico". Your ESTA application will be valid for two years or until your passport expires whichever occurs first.