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."
The former Chancellor Alistair Darling has said the EU referendum is the "biggest decision" the UK will make "in a generation", as he called on people to be more engaged in the debate.
Mr Darling's intervention comes after the German Chancellor Angela Merkel stated that she "personally" hopes Britain votes to stay "part and parcel" of the EU.
Speaking to ITV News, the Remain campaigner said:
Labour leader Ed Miliband has paid tribute to Alistair Darling as a man of "values, decency and kindness" who "distinguished himself as an extraordinary public servant" after he confirmed he will stand down as an MP at the next election.
Mr Miliband said Mr Darling played a "crucial role in helping" Labour win power in 1997 and showed "both conviction and competence" in his various Cabinet roles.
He said the former chancellor could "take pride" that he "helped steer our country through the worst financial crisis to hit the world in living memory", adding: "His was a calm head when calm heads were needed.?"
Mr Miliband said the MP for Edinburgh South West would "always be remembered for leading and winning" the campaign to keep Scotland in the United Kingdom and would be "much missed from the House of Commons".
Alistair Darling has confirmed he will stand down as a Labour MP at the next election and thrown his weight behind Jim Murphy to become the party's new leader in Scotland.
Labour MPs have paid tribute to Alistair Darling following reports the former chancellor will stand down as an MP at the next election.
Conservative MP Matt Hancock also paid tribute to Mr Darling from across Parliament's benches.
Former Chancellor Alistair Darling is set to stand down as a Labour MP at the next election, the Financial Times has reported.
Shadow business secretary Chuka Umunna led tributes to Mr Darling, who led the successful Better Together campaign to keep Scotland as part of the United Kingdom.
The leader of the Better Together campaign, Alistair Darling, has called the result a "momentous result for Scotland and also the United Kingdom as a whole".
He said that it "reaffirmed all that we have in common and the bonds that tie us together - let them never be broken".
But he said the surge in support for independence showed great dissatisfaction and that "every politician must now listen to their cry for change".
Scottish people "do not expect to be shouted down" and want to debate freely, the leader of the Better Together campaign told Good Morning Britain.
While Alistair Darling acknowledge it was only a small minority of Yes campaigners who had intimidated voters, he said it is "not the sort of Scotland any of us want to live in."
The leaders of the Better Together and Yes Scotland campaigns, Alistair Darling and Alex Salmond, are about to take part in a web debate on Mumsnet.
The move is being seen as an attempt by both leaders to reach out to female voters, who polls have shown to be in favour of independence for the most part.
Mumsnet founder Justine Roberts said: "Our Scottish users have been actively debating the referendum on Mumsnet.
“Many are firmly in either the Yes or No camp but a significant number are still very much undecided and we hope the webchat will help them reach a decision on who to vote for.”
“The issue of the economic viability of an independent Scotland has been raised far more frequently than individual household budgets in discussions, but the impact of independence on jobs, welfare, health and education are all areas of concern for Mumsnet users.”