The former foreign secretary warned Brexit negotiations will take longer than the formal two-year negotiating period as MPs debated the issue in the Commons.
Dame Margaret Beckett said she did not believe “for a second” a deal could be done within the time frame.
The Labour grandee said: "These negotiations that we trigger with this Bill will be extraordinarily difficult and they will be very time consuming.
I personally do not think for a second that the negotiations can be concluded within two years and I don't think anybody who has ever negotiated anything would
Dame Margaret Beckett
"I personally do not think for a second that they can be concluded within two years and I don't think anybody who has ever negotiated anything would.
"It will be vital therefore to allow us to make preparation for possible transitional arrangements."
Speaking in the Commons as MPs debate a Bill which would allow Brexit talks to begin, Dame Margaret said she did not intend to challenge triggering Article 50, despite “deeply regretting the decision made by the British people to leave the EU”.
Labour's Dame Margaret Beckett says she does not believe Brexit can be wrapped up in two years
The former foreign secretary said Brexit talks will be "extraordinarily difficult"
Dame Margaret also warned that the UK's decision to ditch Brussels could have "potentially catastrophic" consequences for the nation's economy.
She said: "Though I accept that decision to leave, and I will vote for the bill, I still fear that its consequences both for our economy and our society are potentially catastrophic."
It comes as shadow Brexit secretary Sir Keir Starmer urged his fellow Labour MPs to respect the result of the referendum.
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He said: "Yes, technically the referendum is not legally binding but the result was not technical – it was deeply political.
Britain will go head to head with EU Parliament chief Brexit negotiator Guy Verhofstadt
Shadow Brexit secretary Sir Keir Starmer urged Labour MPs to respect the referendum result
"And politically the notion that the referendum was merely a consultation exercise to inform Parliament holds no water.
"When I was imploring people up and down the country to vote in the referendum and vote to Remain, I told them their vote really mattered, that a decision was going to be made.
"I was not inviting them to express a view and although we're fiercely internationalist, fiercely pro-European, we in the Labour Party are, above all, democrats."
Theresa May's 12 point Brexit plan
Mon, January 16, 2017
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Continued cooperation with the EU to tackle terrorism and international crime.
The debate, which could continue until midnight tonight, is expected to carry on tomorrow before a vote is expected on the European Union (Notification Of Withdrawal) Bill.
Ministers were forced to bring forward the proposed legislation after the Supreme Court ruled that Parliament's approval is needed before the two-year Brexit withdrawal process can begin.
Theresa May has said she wants to trigger Article 50 before the end of March.