Northern Ireland could find itself in a different time zone to the rest of the UK under Boris Johnson’s Brexit deal.
Currently, EU countries and the UK switch to summer time hours on the last Sunday of March and back to Greenwich Mean Time on the last Sunday of October.
But if Europe moves to a “double summer time” arrangement, the prime minister’s Withdrawal Deal could mean Northern Ireland has to be one hour ahead for six months every year.
The European Parliament voted last year to stop changing clocks twice annually.
Mr Johnson’s Withdrawal Agreement commits Northern Ireland to following EU rules on goods and agriculture to prevent a hard border between it and the Irish Republic.
This could mean 1.8 million people in the six counties would follow summer time hours even in autumn when Great Britain winds its clocks back by an hour.
A report by the Lords’ EU internal market sub-committee said: “Were this proposal (for double summer time) 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.”
A group of cross-party peers said the government must give “urgent further consideration” to the impact this would have, criticising ministers’ lax approach to the situation.
They wrote: “Our inquiry has demonstrated that any such decision at EU level would have implications for the UK, notwithstanding UK withdrawal from the EU.
“The nature and significance of such implications is not, however, well understood – not least by the government.”
Baroness Donaghy, chair of the committee, said: “So far the government has stuck its head in the sand on the EU Commission’s proposal, hoping that it goes away.
“However, if it doesn’t, we could be caught unaware and unprepared to make a decision, leaving the island of Ireland with two time zones at different times of the year and causing difficulties for people and businesses in Northern Ireland.”
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.”