Farron said: "Labour's Brexit plan has more holes than a colander".
Starmer said that a Labour government would rewrite May's Brexit negotiating strategy and would focus instead on a deal that retained as numerous benefits as possible of the single market and customs union. "Sensible negotiations start by leaving the maximum number of options on the table".
Broadcasters are facing calls for the Prime Minister to be "empty chaired", after she made clear she will not participate in televised debates in the run-up to the June 8 General Election.
"The White Paper will have a strong emphasis on retaining the benefits of the single market and the customs union as Labour know that is vital to protecting jobs and the economy", Starmer told a press conference on Tuesday.
May has said that she favors a clean break - leaving the single market and removing the United Kingdom from the obligation to allow unlimited migration from the European Union, known as "freedom of movement".
Starmer said the party "genuinely accepts the outcome of the referendum" - a concession that may drive those who want to reverse course into the arms of the more pro-EU Liberal Democrats.
Criticising the government, Starmer added: "Theresa May has said that no deal is better than a bad deal".More news: Senate chairman: Flynn has not responded to subpoena
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Responding to Sir Keir's remarks, Brexit Secretary David Davis said Labour's promise not to walk away from exit talks "really undermines" Britain's negotiating position. We need to recognise that we have 44% of our trade with the EU.
Answering questions during a campaign visit to Bridgend in South Wales, Theresa May dismissed Labour's Brexit proposals as "nonsensical".
However the Conservative Party said that Labour's figures were misleading as Tory councils are smaller in geographical size and tend to have more protected land.
"In a general election campaign you have the opportunity to get policy out to the public in a way that is not normally the case", they said.
And there were signs that his strategy launch was being snubbed by the Labour leadership when a list of election priorities sent to Labour MPs was leaked, revealing that Brexit did not appear anywhere in the top "lines to take".
"It is often said that European Union nationals contribute to our society and so they do, but they are part of our society. and they should not be used as bargaining chips", he said in a speech in London.
"This election is a chance to change the direction of the country and prevent a hard Brexit", said a spokesman for the party.