The UK government must establish a clear "game plan" for a transitional trade deal with the EU as Brexit negotiations begin, according to two influential groups of peers.

The House of Lords' External Affairs and EU Internal Market sub-committees said a temporary extension of the UK's involvement in the EU's customs union could be an "important" element of such an arrangement.

"It is unlikely that a bespoke EU trade agreement can be agreed within Article 50's two-year period, so a transitional deal is vital for protecting UK trade, and jobs that rely on trade," said Baroness Verma, chair of the EU External Affairs group.

"The government should focus on trade with the EU and its World Trade Organisation (WTO) schedules. Deals with non-EU countries are contingent on the outcome of these negotiations, and need to be sequenced accordingly.

"The complexity of the issues and the tight timetable require a significant scale-up in capacity in government departments and clear leadership across Whitehall."

The 13 December report comes just a day after Chancellor Philip Hammond said a transitional deal would be "generally helpful" to help produce a "smoother transition" for the UK's exit from the EU.

"There is an emerging view... that having a longer period to manage the adjustment between where we are now as full members of the EU and where we get to in the future would be generally helpful, would tend towards a smoother transition and would run less risks of disruption," he told the Treasury Select Committee.

But the UK government has refused to give a "running commentary" on its Brexit negotiations with the EU. Prime Minister Theresa May has promised to trigger Article 50, the official mechanism to split from Brussels, by the end of March 2017.

The European Commission's lead Brexit negotiator Michel Barnier said a deal between the parties could be reached by October 2018 and ratified by March 2019.

"We are entering uncharted waters," the French politician said earlier in December.

"The work will be legally complex, politically sensitive and it will have important consequences for our economies and people on both sides of the [English] Channel."

The Department for Exiting the EU had not responded to a request for comment at the time of publication.