Jeremy Corbyn is under mounting pressure to dismiss his policy adviser, Andrew Fisher, as a second, stinging letter of complaint about his past support for candidates from other parties was leaked to the Observer.

Fisher was suspended by Labour general secretary Iain McNicol on Friday, two weeks after Emily Benn, former parliamentary candidate for Croydon South, lodged a complaint to the party, saying Fisher backed the Class War candidate in her constituency ahead of the general election in May.

In a move that angered many at the top of the party, Corbyn said he still “had full confidence” in Fisher who would continue to work for him, though he respected the “integrity” of the general secretary’s office.

But in a new blow to Corbyn’s defence of his aide, the Observer has obtained another letter sent on Tuesday to McNicol from a former treasurer and executive member of the Labour party in Brighton, Peter van Vliet, about separate alleged instances of Fisher backing rival candidates. Van Vliet protested “in the strongest possible terms” that Fisher had encouraged people to back Green candidates before the 2010 general election. The Greens’ Caroline Lucas went on to take the Brighton Pavilion seat from Labour with a majority of just 1,252 votes.

Van Vliet told McNicol he found it “unacceptable that Mr Fisher is now allowed to remain a party member” because he urged people to consider supporting parties other than Labour.

A record of a meeting of the Socialist Alliance organisation in June 2009 notes how it heard “a personal statement from Andrew Fisher of the LRC [Labour Representation Committee] that any left platform should also consider endorsing socialist Green candidates and socialist Labour party candidates”.

After Benn’s complaint, Fisher apologised for putting out a tweet in August 2014 which said: “FFS if you live in Croydon South, vote with dignity, vote @campaignbeard.” @Campaignbeard was the Class War candidate’s Twitter account. Fisher maintained that the tweet was a joke and did not indicate his support for Class War at the time, a line since backed in public by Corbyn’s shadow chancellor John McDonnell.

Labour’s rule book says that anyone who supports a candidate standing against the official Labour candidate “shall automatically be ineligible to be or remain a party member”.

As the row divided party moderates from leftwingers around the leader, who are determined to protect Fisher, Labour MP John Woodcock, former chair of the Blairite Progress group, warned Corbyn not to let the dispute become a “battle of wills”. “It is just not acceptable to employ someone, paying them through the fees of Labour members, who celebrates when respected longstanding MPs such as Ed Balls lose their seats. The situation is beyond ridiculous”.

A former chair of the parliamentary Labour party, Lord Soley, said Corbyn was playing into the hands of the Tories by championing Fisher, who has used Twitter to criticise leading figures in Ed Miliband’s team. Soley told the BBC’s Today programme: “We don’t want people who have been slagging off Labour candidates and telling them not to vote Labour. We are betraying the very people who voted Labour in the hope they would get a Labour government. Going on like this, we will continue to keep the Conservatives in power.

“This is very similar to the position the Tories got themselves into when they had Iain Duncan Smith as leader, and we actually did all we could to keep him there. The Tories are doing exactly the same with Jeremy now.”

Former London mayor Ken Livingstone dismissed complaints against Fisher as a “complete nonsense”. “The people driving this are trying to undermine the leader who has just been elected, and that’s completely unacceptable,” he said. “If you are one of those New Labour MPs who thinks the Blair government was the apex of human civilisation, you have got to come to terms with the fact that the party has moved on.”

A senior Labour figure, a member of its national executive , told the Observer Fisher should be subject to the same rules as everyone else in the party. “The problem is that the rule book does not cover the leader’s office. So someone can, technically, be suspended or even expelled from the party and there is nothing to stop them working for the leader.”

The NEC, including its rules subcommittee, is expected to debate, within the next few days, whether to expel Fisher.