There have been calls among European leaders for the European Union to reform as they continued to react with sadness to Britain's exit.
Italian Prime Minister Matteo Renzi tweeted: "We have to change (the EU) to make it more human and more just, but Europe is our home, it's our future."
Estonian Prime Minister Taavi Roivas said Europe must "work hard" to protect the EU's unity. He said Britain's departure was a "great loss".
Austrian Chancellor Christian Kern warned Britain's exit would harm the other nations, but said he doubted it would trigger a series of referendums among other EU members.
"I do not fear a domino effect," he said. "Europe will lose status and significance in the world because of Britain's step. The long-term economic effects will also be felt for some time."
Foreign Secretary Philip Hammond said there will have to be a "trade-off" on UK access to the EU single market and migration controls.
France's foreign minister called for the UK to trigger the exit clause after a meeting with counterparts of the six EU founding states.
European leaders have reacted to the UK's historic vote to leave the EU.