Yes you can. You will need to enter the number of the voucher into their online application.
The voucher booklet, and I have mine in front of me, says the voucher(s) can be used "in-store, online and at Tesco direct."
After that use, although you will still be physically holding the voucher (and I am not suggesting YOU would try to do this) if you tried to use the voucher when next visiting a Tesco store, it would get rejected. In other words, after the voucher has been used online, put it in the shredder.
