Boris Johnson's Brexit deal could lead to a "time border" in the Irish Sea, according to a House of Lords report.
Last year, the European Parliament backed proposals to stop the one-hour clock change which extends daylight hours in summer across Europe.
Either permanent summer time or winter time would be chosen and the changes would come into effect from 2021.
The House of Lords EU Committee's report - Clock changes: is it time for change? - is released today and states that a time border between Northern Ireland and the Republic would have "enormous practical implications".
Peers have warned that Prime Minister Boris Johnson's Withdrawal Agreement could see Northern Ireland legally obliged to be one hour ahead for six months every year.
It would mean that Northern Ireland would follow summer time hours, even when people in Great Britain wind their clocks back by 60 minutes in the autumn.
Peers have suggested that any EU shift in timekeeping might mean that Northern Ireland will have to follow suit due to the terms of the Prime Minister's deal.
"Were this proposal to become EU law under its current single market legal basis, Northern Ireland may be obliged under the terms of the Withdrawal Agreement and the protocol on Ireland/Northern Ireland to align with the EU and thus institute a time border with Great Britain," said the report.
The group of cross-party peers have called on the Government to "give urgent further consideration" to the impact the Brexit agreement could have if the EU does decide to make the summer time switch permanent.
"Our inquiry has demonstrated that any such decision at EU level would have implications for the UK, notwithstanding UK withdrawal from the EU," wrote the peers.
Business minister Kelly Tolhurst, giving evidence to the committee last year, said: "Anything that would create a time border in Northern Ireland we are completely opposed to, and so is the Irish Government."