The Scottish Greens have labelled the decision to exclude them from the first televised leadership debate of the general election as “outrageous”.
STV will screen a 90 minute programme later, featuring the SNP, Conservatives, Labour and the Liberal Democrats.
However the Greens will not be represented as they currently don’t hold any Scottish seats at Westminster, despite having several MSPs at Holyrood.
The broadcaster said it was “comfortable” that the decision met guidelines set by TV regulator Ofcom.
Ross Greer, co-chair of the party’s executive committee, has written to STV chief executive Simon Pitts regarding the programme.
But BBC Scotland News understands the party have been informed STV stands by its decision.
Mr Greer said: “It is outrageous that a national broadcaster has excluded us from their leaders’ debate despite the Scottish Greens having been one of the five major parties in this country for more than 20 years.”
He added that Scotland’s media should “reflect the range of voices and policies on offer at this election” rather than presenting “limited” options.
An STV spokesman said: “Under the terms of our licences, STV’s programmes must comply with the Ofcom Broadcast Code and its rules around due impartiality.
“We are comfortable that our election coverage, including the leaders’ debate, meets these guidelines.”
In the letter sent to Mr Pitts, the Greens – who were in government with the SNP under a power sharing agreement until it collapsed in April – said none of the four leaders taking part were actually standing as candidates at the election.
The letter also claimed that the Greens’ record in government and policies will be criticised without anyone from the party having a chance to defend them.
Ofcom stated it was an “editorial matter” for STV to decide on.
Appearing on the programme will be the First Minister John Swinney for the SNP, Scottish Labour leader Anas Sarwar, Scottish Tory leader Douglas Ross and Liberal Democrat leader Alex Cole-Hamilton.
A separate election debate on BBC Scotland will be held on 11 June.
It will feature all five leaders, with either Patrick Harvie or Lorna Slater representing the Greens.
Mr Swinney has called for calm debate at the debate, which is the first televised opportunity for parties to clash during the election campaign.
He said he was hoping for a “respectful contest based on ideas to improve the lives of the people of Scotland.”
Mr Swinney added: “In tonight’s debate, and in the election campaign ahead, I and the SNP will set out a genuine alternative to the broken Westminster status quo.”
Scottish Conservative chairman Craig Hoy said that the debate would mean the first minister would face “scrutiny on his fixation with breaking up the UK”.
He added Mr Ross would show how the party would “switch the focus from independence to the issues that really matter”.
Scottish Labour leader Anas Sarwar said the election was “a chance for change that we cannot afford to miss”.
He said: “In tonight’s debate I am not just arguing for myself – I will be fighting for change for every single person in Scotland struggling at the hands of this dysfunctional and incompetent Tory government.”
And Scottish Liberal Democrat leader Alex Cole-Hamilton said he was intending to “pitch our message of hope and change to the people of Scotland.”
STV political editor Colin Mackay will host the programme, which airs at 21:00, from the broadcaster’s Pacific Quay headquarters in Glasgow.