Trade unions need to make P&O Ferries a “toxic brand” after the firm sacked hundreds of workers, to demonstrate to the company that its “disgraceful behaviour” will hit profits, the former leader of the UK’s largest trade union has said.

Len McCluskey, who was general secretary of Unite for 10 years, said that under his leadership the union had sought to use “leverage” against firms who treated workers badly.

He claimed the UK Government would fail to act against P&O, and said the unions needed to deal with the situation to prevent other firms from behaving in a similar way.

Mr McCluskey, speaking to PA Scotland, said: “There is absolutely no doubt the Government should step in and make it absolutely clear that they are not prepared to tolerate it and there will be consequences.

“The truth is the Government won’t step in and therefore, as normal, it is down to the power of ordinary working people.”

Speaking in Glasgow ahead of an event to promote his autobiography, Always Red, the former trade union boss said “the development of leverage, as we developed in Unite, is critical for all unions”.

P&O chief executive Peter Hebblethwaite has already conceded to MPs there was “absolutely no doubt the ferry operator was required to consult with trade unions about the sacking of 800 workers, but the firm did not do this because “no union could accept it”.

Mr McCluskey said: “At the moment P&O know they have broken the law, and that they will be fined, they will have to pay money, they will have to pay compensation to the workforce. But they have shrugged their shoulders and said ‘that’s fine, we don’t mind that’.

“So trade unions need to demonstrate that they are capable of inflicting more damage on a company if these decisions are taken, so the consequences make companies like P&O think ‘perhaps we shouldn’t do this, perhaps there is another way’.

“In my book I make it clear that the development of leverage, as we developed in Unite, is critical for all unions.

“We would seek what leverage could be inflicted on P&O, its clients, its customer base, its shareholders, so the shareholders also understand this isn’t as easy as sacking 800 people and giving them some compensation. That’s what needs to happen.”

Asked if that effectively meant making P&O a “toxic brand”, Mr McCluskey said: “Absolutely. And showing them their toxic brand will lead to them effectively losing a sufficient amount of money to make it impossible to achieve the profits they are wanting.”

He claimed employers “will try to get away with anything if they think it will improve their profitability, whether it’s the disgraceful behaviour of P&O or the fire and rehire tactics that have suddenly emerged”.

Mr McCluskey, a left-winger know as Red Len, stepped down from his role with Unite in August, with his autobiography – which has also been described as a “manifesto for the future of trade unionism” – published the following month.

Speaking about the situation with P&O, the former union boss said while the RMT was “the major union involved in the P&O crisis”, other unions were “watching” and “giving as much support as they can because this needs defeated”.

Mr McCluskey will appear in conversation with Roz Foyer, general secretary of the Scottish Trades Union Congress, at an event in Glasgow on Tuesday evening.