Sir John Armitt, head of the National Infrastructure Commission, told the BBC the commission's modelling figures indicated passenger numbers could drop as much as 25 per cent compared to pre-pandemic levels, with fewer people commuting to and from offices.
However, he urged the Government to "hold their nerve" on spending in case commuter levels bounce back over a longer period of time.
Sir John told the broadcaster: "If you want to avoid congestion in the future, if you want to stick to your zero carbon, then you need to get people back on public transport.
"Therefore for the Government, it's going to be a case of continuing to ensure that public transport is available, whether it's the trains or whether it's the buses, so that people can make use of it as and when they feel relaxed to do so."
In a paper published today the NIC, which advises the Government on in its infrastructure plans, says public surveys and initial responses as the UK edges out of Covid restrictions are not a reliable guide to shifts in mass behaviour over the longer term.
The paper 'Behaviour change and infrastructure beyond Covid-19' says it is too early to assume that forced changes during the pandemic will necessarily result in ongoing shifts in behaviour.
After modelling different scenarios for possible transport trends, the NIC says the average number of public transport trips in the scenario with lowest estimated demand is around 25 per cent lower than in the highest demand scenario over the next 30 years.
But the paper stresses the level of uncertainty about how demand could be affected by other factors such as economic and population growth.
Variation of private car use between the scenarios is less significant but still marked, with a range of 10 per cent between scenarios.
NIC commissioner Andy Green said: “The inconvenient fact is that it is always too soon to know what will happen in the future. Responsible investment decisions require honesty about the huge uncertainties that exist in considering which behaviour changes will remain decades after Covid restrictions are lifted.
"But infrastructure decisions cannot be postponed indefinitely and the scenarios we have set out provide one tool for assessing the best options.
“While there are no easy answers, another tool is to adopt a phased approach to major investments, identifying trigger points for different project stages based on what the data is telling us about long term shifts in how we live and work.”
Last summer, Grant Shapps insisted the Government would press ahead with its transport investment plans despite millions of people choosing to work from home rather than travel by bus or train during the lockdown.
The Transport Secretary says he didn't think the coronavirus pandemic and the months of lockdown where passengers have been discouraged from using public transport would deter people "in the longer run".
And he said upgrades to transport infrastructure were often easier when fewer people are using the network, meaning now was "the time to invest".
It comes as HS2 officially launches its first giant tunnelling machine on Thursday May 13.
The 558ft (170m) long contraption will dig a 10-mile (16km) tunnel under the Chiltern Hills, starting from a site in Buckinghamshire near the M25 motorway.
It will work non-stop for three years and is one of 10 tunnel boring machines (TBMs) being deployed between London and the West Midlands for Phase 1 of the high-speed railway.
Mr Shapps said the launch of the first TBM is "a landmark moment for the project".
He went on: "The work has truly begun on taking HS2 northwards.
"The tunnels these machines dig will ensure the benefits of our new high-capacity, high-speed railway run to the great cities of the North and Midlands, forging stronger connections in our country, boosting connectivity and skills opportunities, and transforming our transport links."
Last year the NIC recommended that the Government should prioritise local rail links over the Eastern leg of HS2 connecting Birmingham with Leeds.