Union baron Len McCluskey was seen holding hands with Jeremy Corbyn's chief-of-staff as the pair celebrated the new year at a luxury Scottish hotel.
The Unite boss, 66, was with the Labour leader's office manager, Karie Murphy, 52, at the Loch Fyne Hotel and Spa in Inveraray.
Photographs of the event, published on political blog Guido Fawkes, may raise questions over a potential conflict of interest.
The pair, who were pictured dining together in August, have denied being intimately involved and maintain they are just friends.
A source suggested the pair could have been taking part in a dance at the event, also attended by Labour's chief strategist Seumas Milne.
Their presence at the hotel's three-day gala costing £429 a head will raise further questions about the influence that Unite, Britain's largest union, has over the Labour Party – which it has long bankrolled.
Ahead of the 2017 general election, the union donated £4.4million to Labour's campaign. Mr McCluskey, who is nicknamed Red Len, has openly joked about his role as the party's 'kingmaker' and his desire to shape its policy, sometimes in conflict with Mr Corbyn.
Concerns have been raised within the party over the Unite leader's growing power with Mr Corbyn's critics calling it a 'takeover by stealth'.
Redhead Miss Murphy was at the centre of a row over Labour's selection of an election candidate in Falkirk in 2013 amid claims of vote-rigging by Unite.
The divorced mother-of-two was temporarily suspended from the party after it was claimed the union had flooded the local branch with members in an attempt to get her the nomination.
Miss Murphy, who voluntarily withdrew from the contest, was cleared by the party of any wrongdoing.
Mr McCluskey, who was married for 25 years, separated from his long-term partner Paula Lace several years ago.
Unite said Mr McCluskey and Miss Murphy were among a group of friends interacting during a new year celebration.
This article has been adapted from its original source.
© Associated Newspapers Ltd.