Former Labour chancellor Alistair Darling has said the EU referendum result must be accepted, but ruled out giving the Government a "blank cheque".
Lord Darling of Roulanish's comments came as the House of Lords debated the European Union (Notification of Withdrawal) Bill for the second day.
The legislation, which would allow Theresa May to trigger the formal two-year Brexit process, has already cleared the Commons unamended with big majorities.
During his speech Lord Darling said none of the negotiations could be conducted in secret, since with 27 other countries involved there would be a running commentary every day.
"Let's be grown-up about it. Let's engage positively," Lord Darling said.
"From the Brexiteers and Government point of view, they have to accept there is a large section of the population in this country, a large membership in the Lords and the Commons, who won't accept some of the extreme arguments being put forward and think the voice of reason must prevail for the good of our country."
Lord Darling also warned the "middle ground of British politics" had been abandoned, adding: "That's a very dangerous place for us to be."
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