Warsaw (AFP) - Poland would like to see Britain hold a second in-out referendum on EU membership, the leader of the country's ruling party said Monday, warning however the bloc would have to "radically change" for Britons to want to return.

"Our idea for today... foresees efforts aimed at making Britain return, including a second referendum," Poland's powerful rightwing leader Jaroslaw Kaczynski said.

But, he cautioned, the "EU would have to radically change" for this scenario to be feasible.

"Britain today risks falling apart", he added, quoted by the Polish PAP news agency.

Kaczynski is the first European leader to make an outright call for a second referendum.

In contrast, other leaders in the bloc have pressured London for a speedy divorce.

Although he holds no cabinet post, as the leader of the governing populist Law and Justice (PiS) party Kaczynski is widely regarded as the government's real powerbroker.

The government swept to power in elections last October on an anti-migrant and populist spending platform.

A strong opponent of European federalism, Kaczynski insists that Britain's vote to leave meant it was time for a "new European treaty" to guarantee the rights of nation states.

Describing Brexit as something "very bad", Kaczynski also warned on Monday that pushing ahead with the old Franco-German model of ever-closer political and economic integration could "end in catastrophe".

He called for EU institutional reforms that would make Europe "a superpower," but based on a confederation of nation states under a president in charge of a powerful military.