Nato chief warns Brexit could increase 'instability'

"What really matters for Nato is, I strongly believe, a strong UK in a strong EU. It is good for both the UK and the EU but also for Nato," Stoltenberg, the head of the 28-member military alliance, said at an event on Monday (6 June) organised by news organisation Politico.

"It is a great advantage to have the UK being strong in the EU, pushing for security cooperation with Nato. It's good to have the UK in Nato pushing for strategic cooperation with the EU.

"We live at a time of many threats and instability, we don't need more instability, we need more cooperation in Europe." He declined to specify what type of instability he was referring to.

The British will vote on 23 June on the UK's membership of the European Union. Tuesday is the last chance to register to vote.

Leave camp grows stronger

According to polls published on Monday, supporters of Brexit edged ahead, putting pressure on prime minister David Cameron who campaigns for the UK remaining in the bloc.

The WhatUKThinks average of the six last polls put "Leave" at 51 percent against "Remain" at 49 percent.

Campaigners for the Remain camp argue leaving the EU would put the UK's economy in danger.

Cameron on Monday evening argued in a debate that leaving the EU would "put a bomb under our economy".

Another central theme in the campaign is the question of EU workers from eastern EU states living in the UK.

Boris Johnson, former mayor of London, a Conservative supporter of the Leave camp claimed on Monday: "We have absolutely no control over the people coming from the whole 27 other EU countries, some of them with criminal records."

In the meantime, 10 trade unions have urged their members to vote to remain in the European club, claiming that parental leave and holiday rights would be under threat from a Conservative government without EU protection.