The Liberal Democrat manifesto will include a commitment to another EU referendum where the party would campaign for Remain, Nick Clegg says.
The former deputy prime minister said that once talks are complete, voters should get two choices – accept the Brexit deal or stay in the EU.
Setting out his party’s Brexit position, he claimed leaving the EU would damage the UK economy by £59bn.
Theresa May says she is determined to make a success of Brexit.
She says a separation deal can be reached that is good for both the UK and the EU.
But Mr Clegg, who is the Lib Dems’ Brexit spokesman, said voters should be able to “judge for themselves whether it is the right future for the country” once talks are complete, which is expected to be in 2019.
“That referendum will offer two choices; accept the deal, or remain in the EU. Liberal Democrats will campaign for a Remain vote,” he said.
Mr Clegg said the PM had chosen to “pursue the most extreme and damaging form of Brexit”, adding that the fall in the value of sterling would be sharply felt by holidaymakers travelling abroad.
Despite many gloomy pre-referendum economic forecasts having not materialised, Mr Clegg said the “long-term impacts won’t start to be felt until 2019 at the earliest”.
He claimed a “triple whammy of higher inflation, lower business investment and lower net migration” means that GDP is likely to be 2.4% lower in 2021 than it would have been without the EU referendum.
As a result of extra borrowing, Brexit will have “dented the public finances by £59bn over a five-year period”, he predicted.
Mr Clegg also launched an attack on his former partners in government, saying the economic damage from Brexit was “already being felt by the people who the Conservatives have always cared about least: the poor, the insecure and the vulnerable”, and that Mrs May had “specious excuses” for calling next month’s general election.
A Conservative spokesman said: “Every vote for Theresa May gives her a better hand in Brexit negotiations, helping to get a deal which strengthens our economy and helps UK consumers.”