The former top official in Britain's trade ministry on Tuesday compared the government's plans for EU ties after Brexit to swapping a three-course meal for a packet of crisps, ahead of a

major speech defending the policy by his ex-boss.

Martin Donnelly, who was the senior civil servant at the Department for International Trade until March last year, criticised plans to leave the EU's single market after Brexit in March 2019.

"You're giving up a three-course meal, which is the depth and intensity of our trade relationships across the European Union and partners now, for the promise of a packet of crisps in the future," he told BBC radio.

He added that any trade deals struck outside the EU "aren't going to compensate for what we're giving up. You just have to look at the arithmetic -- it doesn't add up I'm afraid."

Donnelly's comments came just hours before his former boss, International Trade Secretary Liam Fox, was due to make a major speech about the advantages of pursuing an independent trade policy.

Fox, a leading advocate of Brexit in the 2016 EU referendum, will criticise the opposition Labour party's new support for staying in a customs union with the bloc.

Such a move would mean accepting EU rules on trade in goods and would limit Britain's ability to forge outside trade deals and new development policies, he will say.

- 'Complete sell-out' -

"As rule takers, without any say in how the rules were made, we would be in a worse position than we are today," he will say.

"It would be a complete sell-out of Britain's national interests."

Citing Turkey, which has a customs union with the EU, he will say such an agreement "would remove the bulk of incentives for other countries to enter into comprehensive free trade agreements with the UK if we were unable to alter the rules in whole sectors of our economy".

"The inevitable price of trying to negotiate with one arm tied behind our back is that we would become less attractive to potential trade partners and forfeit many of the opportunities that would otherwise be available to us," he said.

Fox's speech follows a string of Brexit addresses by top British ministers, culminating with one by Prime Minister Theresa May on Friday, when she is expected to set out further details on future trade ties.

Talks with the EU on the post-Brexit partnership are due to begin in April, with London hoping for a deal by the end of the year.

Labour leader Jeremy Corbyn on Monday backed a new customs union after Brexit, a move that some members of May's Conservatives have also been pressing for, and is endorsed by the CBI business group.

He said it would ensure British manufacturers could continue to trade freely with businesses on the continent, while also helping avoid any return to border checks on the Irish border.

Britain is currently part of the EU's customs union, which has a common external tariff on imports, allowing goods to move freely inside the area.

But May has repeatedly said Britain will leave in order to sign its own trade deals, and her spokesman confirmed on Monday that the government "will not be joining a customs union" after Brexit.afp