TOP legal experts warned last night Theresa May’s Chequers Brexit plan could be illegal.

Expert Professor David Collins said there was a major risk of the UK breaching World Trade Organisation rules on tariffs and trade.

After examining the PM’s White Paper he said there was a “serious” chance “at least two” parts of the Facilitated Customs Arrangement (FCA) that would break international trading laws.

Prof Collins said the worst case scenario was that the FCA would have to be abandoned, or significantly modified.

He said: “It is difficult to see what modifications could successfully address these potential illegalities without destroying the whole purpose of the FCA”.

The academic said the PM’s blueprint would also “severely constrain” the UK’s ability to sign free trade agreements with other countries.

He added: “It is hard to believe that a third country would agree to offering lower tariffs to UK exports when that third country’s exports to the UK would be subject to a higher EU tariff at the border, subject to the promise that there will be a rebate at a later stage pursuant to a cumbersome tracking procedure.”

And he warned foreign traders would find the system overcomplicated meaning they could decide against exporting to the UK and EU because of the confusing tariff system.