Peter Mandelson has said he will step down as European Union Trade Commissioner in 2009, but told the BBC he could not be sacked before then.
Mr Mandelson has said relations were strained after the 1994 leadership race
It follows speculation that he may lose his job if, as expected, Gordon Brown succeeds Tony Blair as prime minister.
But he said: "I don't know whether this is going to come as a disappointment to him, but he can't actually fire me."
He and the chancellor reportedly fell out when he backed Mr Blair for the Labour leadership in 1994.
The former Northern Ireland Secretary remains a close confidant of Mr Blair.
Last year he said a "deep breach" had opened up between him and Mr Brown when he had backed Mr Blair to become the new leader and it had created a "fissure" in New Labour from the start.
When Mr Mandelson was appointed EU Commissioner after being forced out of two Cabinet posts, it was widely reported Mr Brown was delighted to see him sent off to Europe.
But as Mr Brown is increasingly seen as the successor to Mr Blair, who is due to step down as prime minister before September, there has been speculation Mr Mandelson's European career could be cut short.
Asked whether he would keep his job under Mr Brown, Mr Mandelson told BBC Radio 4's The World At One: "I don't know whether this is going to come as a disappointment to him, but he can't actually fire me.
"So like it or not, I'm afraid he will have to accept me as Commissioner until November 2009.
"But I will not be seeking a nomination for a further term after that time."
Mr Mandelson has called Mr Brown the "natural successor" to Mr Blair.
And he told the BBC the chancellor - who was this week criticised by a former colleague for "Stalinist ruthlessness" - would be able to work with other European leaders.
"I think he has the very real potential to be a constructive European, but it takes working at, and I'm sure he would be the first to acknowledge that," said Mr Mandelson.