The UK should include international shipping emissions in its 2050 climate targets to slash its greenhouse gases.
The Committee on Climate Change's chief executive David Kennedy said emissions from shipping needed to be part of the overall goal to cut greenhouse gases by 80 per cent by 2050 because they were "too big and too worrying to forget about".
A report from the committee estimates that with efforts to reduce emissions from other areas such as the power sector, by 2050 shipping could account for up to 11 per cent of the total greenhouses gases the UK is allowed to emit under the legally-binding targets in the Climate Change Act.
"Our report highlights the high degree of uncertainty over current and future shipping emissions and the need to resolve this," he said.
"It is clear that shipping emissions could well be significant, and so cannot be ignored - they should be included under the Climate Change Act."
Currently neither international shipping nor aviation are included in the long-term targets to cut greenhouse gases or the five-yearly emissions limits known as carbon budgets.
Analysis by the committee shows that emissions from shipping, calculated on the basis of the distance travelled by all vessels journeying to the UK and their fuel efficiency, accounts for around 12-16 million tonnes of greenhouse gases.
That is currently a small percentage of the 570 million tonnes emitted by the UK as a whole, but as emissions fall under efforts to move the UK to a low-carbon economy, shipping's share of total greenhouse gases will grow.
While the International Maritime Organisation, the international shipping regulatory body, has developed policies to cut emissions from the sector, the Committee on Climate Change said there was significant scope to be more ambitious.
A range of technical and operational measures could be put in place, such as using towing kites to allow ships to use wind energy, installing solar panels to reduce energy use, cleaning hulls more often to reduce drag and cutting the speed at which the vessels travel to save fuel.
Biofuels could also have a role to play, along with liquefying natural gas which has a lower carbon output than conventional fuels.
Mr Kennedy said international action such as an emissions cap and trade scheme or a global levy on fossil fuels would encourage the shipping industry to bring in measures to cut their greenhouse gases.
If there is no progress on a global deal to tackle shipping emissions, the EU has said it will move the sector into its emissions trading scheme, something which Kennedy said was not ideal but might be a second best option.
"It is clear that there is scope to reduce emissions, which would reduce costs of inclusion," he added.
"In order to ensure this, the government should proactively support developments of new policies aimed at encouraging investment in cleaner shipping technologies and more efficient operational practices."
The committee will provide formal advice to the government on including shipping and aviation in the carbon budgets next spring.
Friends of the Earth's transport campaigner Richard Dyer urged the Government to include shipping in the climate targets.
"Ignoring the growing climate impact of shipping would be a titanic mistake which could sink our ability to develop a safe and prosperous future," he said.
"The international community must also take urgent action to ensure the shipping industry plays its part in a low-carbon transport system."
David Balston, a director at the UK Chamber of Shipping, a trade group, welcomed the committee's work and agreed that international shipping should be included in UK carbon budgets.
"We do stress, however, that any solution must be global rather than regional to avoid distorting world trade and potentially damaging an industry that is vital to the future prosperity of the United Kingdom," he said.