Industrial relations in Glasgow have reached a "new low" after school janitors were threatened with formal proceedings if they boycott duties over a pay dispute, a union has said.

Cordia, an arms-length organisation (Aleo) of Glasgow City Council, last week sent letters to workers warning them of the "potential consequences" of the boycott, which begins on Tuesday.

The letters state that janitors who take part in the boycott could be determined to have "failed to undertake a reasonable instruction" and would be subject to formal proceedings initiated by management.

It comes after 98% of janitors in Glasgow who are Unison members voted to take action short of a strike. This means they will no longer carry out weeding, litter picking, snow clearing, salt spreading, leaf clearing, heavy lifting, spillage clearing and animal clearing from January 19.

The decision, which involves around 130 workers, was made after Cordia refused to pay a working context and demands payment to school janitors. Under Glasgow Council's Aleos pay structure, any workers who undertake duties which are dirty, unpleasant, involve working outside on a regular basis or heavy lifting are eligible for the payment.

Unison has written to the council asking them to intervene and have the letter withdrawn. However, the union said they had not received a reply.

The letter, addressed to council leader Frank McAveety, states: "Cordia has today lettered Unison members threatening them with individual 'formal proceedings' if they participate in Unison's official industrial action.

"This is beyond words to be honest, and not in keeping with the Council's commitment to oppose the Tory's anti-trade union bill. A new low for industrial relations in Glasgow.

"Can I ask that you intervene to have the letter withdrawn via the same communication routes it was circulated?"

Unison says it contacted Cordia earlier in January in a bid to reopen negotiations, but Aleo did not reply.

A spokesman for the union added: "Jeremy Corbyn and John McDonnell, the UK Labour Party leaders, say that they 'automatically support workers taking official industrial action'.

"However, Glasgow's Labour Council is threatening to suspend school janitors taking official action.

"The council needs to tell Cordia to get round the table and address our members' legitimate claim, not help them make threats against trade unionists."

Cordia could not be reached for comment.