Latest developments:
Labour's deputy leader, Tom Watson, is expected to hold emergency talks with trade union leaders to try and get them to withdraw their support for Jeremy Corbyn.
In a bid to break Labour's deadlock - Jeremy Corbyn refuses to resign as leader, but many of the party's MPs do not support him - sources said union leaders had "reached out" to Mr Watson to see if they could find a negotiated settlement.
However, due to "logistical" difficulties, it was "proving tough" to arrange a meeting between Mr Watson and union chiefs, a source close to the MP for West Bromwich East said.
While Unite leader Len McCluskey said on Sunday that unions could act as "honest brokers" in the leadership saga, leaders of some of the other biggest unions affiliated to Labour have not commented, and some officials are reported to be on holiday.
The leader of the UK's biggest trade union has said the country is 'riven' as a result of the EU referendum.
Len McCluskey told ITV News that the trade unions can help broker a peace deal and reunite the party now the "coup has failed".
Labour leader Jeremy Corbyn said he would 'reach out' to MPs that oppose his leadership but said they should 'respect democracy'.