Speaking near Traitor's Gate, at the Tower of London, Robin Tilbrook said a deal between the two parties would be "against the English".
He also criticised UKIP for not being interested in English nationalism.
The party is fielding 35 candidates at the general election.
In the past it has campaigned for an English Parliament but now says it wants to see a fully independent England, as well as withdrawal from the EU.
The party chose the Hung, Drawn and Quartered Pub, near the Tower of London, to launch its campaign, saying it would send a "message from history for those that might want to conspire against English interests", said Mr Tilbrook.
"We say that the prospect of a Labour/SNP government, perhaps backed up by Plaid Cymru - and there has been some talks, we gather, with Sinn Fein... will be an anti-English conspiracy of a government and therefore we can start to talk about treason," he said.
The SNP has said it is prepared to work with Labour - perhaps as part of a "progressive alliance" with Plaid - to keep the Conservatives out of government, although Labour has ruled out a formal coalition and has described links with Sinn Fein as "absolute rubbish".
Plans to film an interview with Mr Tilbrook at Traitor's Gate had to be abandoned after officials from the Tower put a stop to it.
But the party, which claims to be the only one offering a "voice to the English" at the general election, was able to unveil a defector from UKIP.
Graham Moore said he had been due to stand for Nigel Farage's party at the general election but jumped ship at the start of the year because UKIP "have no interest in England whatsoever".
He is now standing for the English nationalist party in Erith and Thamesmead, east London.
The English Democrats are fielding fewer candidates than they did at the 2010 general election but are concentrating on areas where they have support, including a full slate in South Yorkshire.
They are also standing in the border town of Berwick-upon-Tweed, the most northerly constituency in England, and Monmouth, in Wales, where they are campaigning for a local referendum on whether the area should become part of England again.
Mr Tilbrook said the party was "consolidating our position as the English nationalist party", and wanted to be ready to capitalise if UKIP's progress came to a "juddering halt" at the election, offering a home to "those people in UKIP who are basically English nationalists".
The party is also campaigning against Labour, Conservative and Lib Dem plans to give more self-government to the English regions, arguing that it would a "shame" to sacrifice England's unity for a "last gasp attempt to save the Union".
The leader of the English Democrats warned about an "anti-English conspiracy" between Labour and the SNP as he launched the party's general election campaign.