Raw water from Sungai Semantan will be diverted into the tunnel and channelled into Sungai Langat for treatment in seven existing plants.
The proposal is among the suggestions from the national action committee under the Energy, Green Technology and Water Ministry.
It was expected to be presented to the Cabinet as Tunnels went to press.
The tunnel, which is almost completed, was originally designed to channel water directly to the Langat 2 water treatment plant, delayed by three years now.
The Pahang-Selangor Raw Water Transfer Scheme includes the tunnel and the two water treatment plants.
The proposed use of the tunnel reflects the urgent action the Federal Government realises to ease the burden of 6.7 million consumers in Selangor, Putrajaya and Kuala Lumpur since February.
Minister Datuk Seri Maximus Ongkili confirmed on 14 April that he would submit several proposals drawn up by the committee headed by his deputy Datuk Seri Mahdzir Khalid.
"The panel’s proposals can be put in place to overcome the shortage," he said without giving details of the other suggestions.
Ongkili said the government would consider all possible measures to resolve the crisis before the situation worsened, adding that the water level in the Sungai Selangor dam, which supplies 60 per cent of raw water to treatment plants, was now at 37 per cent.
"If rain does not fall in catchment areas over the next two weeks, the level in the dam will drop to 30 per cent. The Federal and Selangor governments need to search for urgent steps to ensure the demand for water is met," he added.
However, it is understood that even if the plan to use the tunnel sooner is approved by the Cabinet, it could take six months to implement.
"There are other things to consider before water can flow through the tunnel. One of the things is to remove construction machinery and to conduct checks on the structure," said an official familiar with the proposed plan.
According to the building plan, the tunnel is supposed to be able to transfer up to 1,890 million litres of raw water daily but it has yet to be determined how much of water will be channelled under the contingency plan.
Consumers in Selangor, Kuala Lumpur and Putrajaya are using 4,351.33 million litres per day.
The total production capacity of 34 treatment plants is 4,686 million litres per day.