​Brexit will “undoubtedly” damage Britain’s capacity to work with other EU countries on issues relating to foreign affairs and its ability to exert influence in the world, according to the former Conservative Foreign Secretary William Hague.

Speaking to the House of Lord’s EU external affairs sub-committee, Lord Hague, who is also a former Tory leader, added it could even be possible Britain would be in a position of advocating EU membership for countries in the Balkans while simultaneously negotiating to leave the bloc.

But when asked by peers how Brexit will affect the UK’s relationship with the 27 other EU countries, he replied: “Brexit will be damaging to our ability to work with other EU countries, obviously on foreign affairs and influence their outlook overall.

“There is no doubt we are a big player in the foreign affairs council, bigger than we are in the financial and economic affairs of the EU. It is also true that they will want to consult us, to temper the gloom a little bit.”

Lord Hague added that Britain should seek permanent membership of the EU’s Common Political Security Committee which he said was “very important to the UK”. An arrangement of this kind would “mitigate the damage that will undoubtedly be caused”, he told peers.

The committee “amplifies the UK's weight in the world” by enabling it to lead a united response on issues like sanctions on Iran and safeguards of British interests in areas such as the Falklands, where Argentina recognises the “solidarity among 28 countries” that will be shown in response to any hostile actions, he said.

Former defence secretary and Nato secretary general Lord Robertson told the committee that the reduction in UK influence also came at a moment when Europe was having to take a bigger security role as the US downgraded the importance of the transatlantic partnership.

Cathy Ashton, the former EU foreign affairs representative, added: “The big and most fundamental issue is that in the process of working up the way in which policy is developed and determining what Europe’s policy would be, we won’t be in the room to either influence it in one way or another. That will be a loss to the European Union and potentially a loss to our capacity to develop policy.