Andrew Fisher has written to Iain McNicol, General Secretary of the Labour Party, to “completely and unreservedly apologise” for his Tweet suggesting people in Croydon South vote for the Class War candidate in the general election.

Jeremy Corbyn’s political adviser said the Tweet, sent in 2014, was intended as a joke. “I obviously do not and did not support Class War in any way, let alone in an election,” he said. “I was and am committed to the election of a Labour government and a successful and united Labour Party.”

The defeated Labour candidate in Croydon South, Emily Benn, has written to McNicol asking for the National Executive Committee to consider expelling Fisher from the Labour Party. Her letter cited the party’s rule book, which says anyone who supports someone standing against a Labour candidate “shall automatically by ineligible to be or remain a party member”.

Fisher’s Tweet, posted in August last year, said: “FFS if you live in Croydon South, vote with dignity, vote @campaignbeard.” @campaignbeard was the Twitter account of the Croydon South Class War parliamentary candidate Jon Bigger.

Fisher’s letter to McNicol says: “I accept that the tweet has been misinterpreted and has caused embarrassment and understandable upset among party members, which I regret. I wish to completely and unreservedly apologise for this tweet. I have now closed my social media accounts and assure you there will be no repetition of such activity in the future.”