JIM Allister showed "homophobic prejudices" during a row over a rainbow Union flag, the Ulster Unionist leader has said.
Ulster Unionist members including MLAs Mike Nesbitt and Doug Beattie and former MP Danny Kinahan, were pictured with a rainbow version of the Union flag as they attended Saturday's Pride parade.
Mr Allister hit out at the picture and wrote on Twitter that it was a "shameful perversion of the Union flag".
He later said: "By overlaying, or infecting, the Union flag with LGBT’s rainbow colours, great disrespect was shown to our national flag in an attempt to misuse it in support of LGBT demands."
He added: "This ill-judged display by UUP elected representatives has rightly caused unease among some UUP members and supporters. I believe it is important that Robin Swann clarifies where he stands on such emasculation of our flag."
His comments drew an angry response from UUP leader Robin Swann who said members of the British Armed Forces recently took part in the London Pride parade carrying a pink version of the Union flag.
"Using words such as 'infecting the Union Flag' demonstrates Mr Allister's own homophobic prejudices," he said.
"He should explain exactly what he means by that and also his claim about the 'emasculation' of the Union Flag."
On Saturday, Taoiseach Leo Varadkar said Northern Ireland's biggest parade is not orange or green, but "rainbow-coloured".
The Fine Gael leader, who attended the parade for the first time, joined tens of thousands of people as they made their way through the city centre.
His presence will be seen as significant as same-sex marriage remains a contentious political issue in the north.
Addressing a huge crowd that gathered in Custom House Square, Mr Varadkar - the first openly gay taoiseach - said the Pride parade is Northern Ireland "at its best".
"Open, inclusive, diverse and for everyone," he said.
"I want to say how great it is to be in Belfast today.
"I always say the biggest parade that happens in Northern Ireland isn't orange or green, it's rainbow-coloured. It's really great to see it today.
"I had a real honour today to walk with Lord Hayward, who, along with Conor McGinn, put the legislation through the Commons and Lords to bring about marriage equality here in a few months' time. So, we really want to thank them."
Northern Ireland remains the only part of the UK where gay marriage is illegal.
That could change, however, after landmark legislation was passed by parliament which will allow same-sex marriage if devolution is not restored by October 21.
Belfast Lord Mayor John Finucane helped raise the first Pride flag flown at City Hall.
"This is a great day in Belfast and for the first time we are marching under the Pride flag which flies over City Hall," he said.
The theme of this year's parade was "rights now", to highlight rights demanded by the LGBTQ community.
Among the tens of thousands of participants were members of the PSNI, An Garda Siochana, Ulster Rugby, BBC Pride, Ulster Bank and Antrim GAA club.
Meanwhile, the head of BBC Northern Ireland addressed the "confusion" around staff members attending Belfast Pride saying the broadcaster was not participating corporately.
The BBC has faced questions over impartiality since announcing staff in the organisation's BBC Pride group would be attending.
BBC NI director Peter Johnston acknowledged there had been "confusion" about the "terms and basis for BBC staff involvement in the Belfast Pride parade 2019".
He said while members of the BBC Pride staff network - an employee-led initiative - would be taking part, BBC NI as a corporate body would not.
"We know that there are legislative issues specific to Northern Ireland in relation to same-sex marriage," he said.
"The BBC's editorial guidelines provide clear advice in this regard. It is on this basis that BBC NI will not be involved corporately in the Belfast Pride parade and that individual programme brands will not be represented."
UTV staff members also took part in the parade.
