Minister for Transport Paschal Donohoe has warned Luas management and unions that the Government will not make additional funds available to meet SIPTU's pay claim for increases of up to 53%.

90,000 people who normally use the Luas face disruption tomorrow as staff begin a two-day strike.

The strike will start at 5.30am with normal services resuming on Saturday, when trams start running at 6.30am.

Another two-day strike is scheduled for 18 and 19 February, and further stoppages have not been ruled out.

Luas workers say they have been underpaid for years and defend their pay claim averaging 40% - but as high as 53% - in some cases.

However, Luas operator Transdev says the claim is unsustainable with the company losing €700,000 last year.

The industrial action has already cost workers their annual 6.5% bonus.

The company has told the 90,000 daily users that while they cannot use their Luas tickets on other public transport services, they can claim a refund for the strike days.

Mr Donohoe warned that the dispute must be resolved within the existing contract between Transdev and the Government, and that the State will not be providing extra funds to meet the claim.

He said the operating contract between Transdev - a private company - and the Government cannot be amended mid-flow - as under-bidders could potentially take action.

Speaking on RTÉ's Morning Ireland, Mr Donohoe said massive efforts have been made to try to resolve the planned strike.

Efforts began in 2014 and the Workplace Relations Commission has been involved in the dispute since last year, he said, adding that he expects the strike to go ahead tomorrow.

The minister urged both sides to return to the Labour Court.

Transdev Managing Director Gerry Madden urged employees to challenge the strategy of their union representatives, saying the dispute will not be resolved unless "sensible voices emerge".