LONDON, Nov 7 (Reuters) - Britain's opposition Labour party must "pull together" after reports of serious internal discontent about its leader, Ed Miliband, or risk damaging its chances at a national election next year, the party's campaign co-ordinator has warned.

Labour have a narrow lead over Prime Minister David Cameron's Conservatives in opinion polls ahead of an election in May 2015. But Miliband's personal rating has sunk to its lowest ever level, raising doubts among supporters whether he is capable of delivering an outright win.

Newspaper reports on Thursday said unnamed members of Miliband's team feared that lawmakers in his own party were circulating a letter calling for him to resign. Miliband dismissed the reports as "nonsense."

Douglas Alexander, the party's general election co-ordinator, called for unity after the plot whispers.

"He (Miliband) has got challenges but all of us have got challenges in every political party and everyone of us in the Labour party has to reflect the reality that divided parties lose elections," Alexander told BBC TV on Thursday night.

"We have got a profound responsibility ... to pull together to offer credible change in the face of these tough economic times and that's exactly what we plan to do." (Reporting by Andrew Osborn; Editing by Guy Faulconbridge)