Speaking at a debate in the European Parliament in Strasbourg, Farage said May was in a "weak position" as she prepared to enter Article 50 talks with the EU.
He was standing in a debate ahead of an EU leaders' summit which will take place on the anniversary of the Brexit vote.
Farage warned the EU that it should not consider the UK election result as a "victory", because the majority of voters supported parties backing the UK's exit. And he called on the Prime Minister to commit to leaving the Single Market and the Customs Union when she meets EU leaders later this month.
The summit is set to be a tense one for May. It will be the first time she meets her EU counterparts since the UK election. The meeting will cover topics such as migration and jobs, but May will be excluded from an after-dinner discussion about Brexit between the leaders of the EU27 nations.