Following a lengthy cabinet meeting in which a number of senior ministers criticised the deal, Ms May met with DUP leader Arlene Foster and then Jeremy Corbyn.
Now, Labour's Brexit secretary has confirmed that the party would be voting against Ms May's agreement.
Sir Keir said the deal was "not good enough for manufacturing and services" and also did not have the necessary safeguards to protect rights.
"We said we would judge the deal once we had seen it. We have now read and analysed the 500 or so pages. It is a miserable failure of negotiation,” Sir Keir said on Sky News.
“There is huge detail on issues like the backstop that the government says it does not intend to use and on then on the question on what you intend to use and the future relationship there is seven pages.
“…We will vote against this deal because it does not meet our tests. I don’t think anybody would sign up to a document without knowing where it was heading.”
Sir Keir added that he believed the agreement would make striking future trade deals more difficult.
The biggest issues facing UK on leaving EU
Show all 8
The comments come as it was announced that MPs could be given the opportunity to table amendments on the governments' motion.
Sources who spoke with Ms May suggested they believed that ministers were heading towards agreeing that MPs will be able to vote on amendments before voting on the deal itself.
The development raises the prospect that MPs may only back the deal if certain conditions are met - for example, that there is a Final Say referendum.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies