The independent-minded left-winger, who has opposed the Government's policy on Iraq, Kosovo and devolution, has held his seat for nearly 40 years despite high-profile SNP attempts to oust him. He is expected to announce his retirement publicly within weeks.
Mr Dalyell, 66, an Old Etonian who still lives in part of what was once his family's castle, has never been afraid to take controversial positions at odds with his party.
Recently, to the annoyance of Labour Party managers, he announced he would no longer vote in Westminster on issues that only affect England. He said: "I am trying to bring it home to the party that they have a problem which is disguised by the size of the majority. The whips will not lose a wink of sleep about the fact that I am not voting on certain issues, but they might lose rather more sleep if the majority was, as it has been in the past, minus one."
In 1977, Mr Dalyell was the first politician to pose what has become known as the West Lothian Question (after his then constituency). This asked whether, once a Scottish Parliament was in existence, a full quota of Scottish MPs at Westminster could justifiably vote on purely English affairs.
During the air strikes on Iraq, Mr Dalyell said that he was ashamed of Mr Blair as a Labour leader. But he reserved his most bitter attack on the Prime Minister for a face-to-face confrontation at a private meeting last month of the Parliamentary Labour Party.
He told him that traditional Labour supporters would be permanently alienated by the "Napoleonic Blair presidency", adding that activists who had "sweated their guts out" to deliver Labour's landslide victory in 1997 had deliberately stayed at home for last month's European elections.
Earlier this year he lamented: "The House of Commons is atrophying. In 36 years I have never known membership of the House being so marginalised."
Given Mr Dalyell's reputation, the news that he may be stepping down has intrigued Westminster. But Mr Dalyell, himself, declined to comment. He said: "This will be discussed, and the first to know will be the Linlithgow Constituency Labour Party at our annual general meeting on 15 August."
Meanwhile, the Scottish MP Tommy Graham, who was expelled from the Labour Party last September for bringing it into disrepute, yesterday vowed to stand as an independent in the next General Election.
Mr Graham represents Renfrewshire West and was disciplined after an internal inquiry when he was mentioned in the suicide note of Gordon McMaster, a neighbouring MP. Yesterday, he said: "The knuckledusters are on. The Labour Party - I will get them back in my own way."
He hopes to emulate Dennis Canavan, the left-winger who won his Falkirk West seat by a landslide at the Scottish Parliament elections after being expelled by the party. But Mr Graham, who has a majority of more than 8,000, is likely to face a tougher battle than the highly popular Mr Canavan.Reuse content