"Desperate" Britain is "delusional" to expect a favourable trade deal with the US, an ex-Obama advisor has warned, as Dominic Raab seeks to "fire up" the UK's trade relationship with North America.

Larry Summers, ex-director of the National Economic Council in president Barack Obama's administration, said Britain has "no leverage" in any trade negotiations and should not hope to be prioritised in a trade deal with the US.

The comments come as Foreign Secretary Raab embarks on his second major overseas trip since being appointed by Boris Johnson, starting in Toronto on Tuesday before moving on to Washington on Wednesday and Mexico City on Thursday.

Mr Raab said ahead of the trip: "In my first fortnight as Foreign Secretary, I'm travelling east and west to underline that the UK is determined to strengthen our friendships with countries across the world and raise our international horizons.

Brexit-supporting Dominic Raab is the new UK Foreign Secretary after replacing Jeremy Hunt. Credit: PA

"I'm determined that we fire up our economic relationships with non-European partners.

"That means working with them now to ensure a smooth transition of our trading arrangements after Brexit and means quickly moving to wide-ranging trade deals that boost business, lower prices for consumers and respect our high standards.

"I also want to build a stronger alliance to uphold international rule of law and tackle the issues that threaten our security, whether that's Iran's menacing behaviour or Russia's destabilising actions in Europe, or the threat from terrorism and climate change."

However, speaking to BBC Radio 4's Today programme, Mr Summers, who was also Bill Clinton's treasury secretary, said: "I'm not sure what Britain wants from the United States that it can plausibly imagine the United States will give.

"If Britain thinks that the American financial regulators who have great difficulty coming together on anything are going to come together to give greater permissions and less regulation of UK firms, I would call that belief close to delusional."

Larry Summers worked under both President Clinton and President Obama. Credit: AP

He added: "Look at it from America's point of view: Britain has much less to give than Europe as a whole did, therefore less reason for the United States to make concessions. You make more concessions dealing with a wealthy man than you do dealing with a poor man.

"Second, Britain has no leverage. Britain is desperate. Britain has nothing else. It needs an agreement very soon. When you have a desperate partner, that's when you strike the hardest bargain. The last thing you do is quit a job before you look for your new one.

"In the same way, establishing absolutely that, as a matter of sacred principle, you're leaving Europe has to be the worst way to give you leverage with any new potential partners."

He said it is "close to inconceivable" that the UK would be able to increase its trade with the US enough to make up for lost trade with the EU.

Former Tory leader Iain Duncan Smith, however, said: "This is a classic attempt by Larry Summers to use Brexit for domestic point-scoring.

"Forty-five Republican senators have signed a letter to the Prime Minister pledging to back a trade deal with Britain once we have left the EU. The President (Donald Trump) himself has expressed his enthusiasm for a UK-US deal.

"If, as Mr Summers suggests, we were only offered a bad deal by the US, we would not accept it. Trade deals must be mutually beneficial to be acceptable to both sides.

"The Democrats, fortunately, are not in control of US trade deals."US Senator Tom Cotton, however, said Britain should be at the "front of the queue" for a trade deal with the US.

Mr Cotton told Today: "Many of my colleagues in the Congress would say that Great Britain should be in the front of the queue given everything our nations have gone through together.

"Obviously it wouldn't be a matter of days or weeks for such negotiations, it might be months, but I would suspect it would be months not years."

Mr Raab said there was a "consistent warmth" for Britain and a "desire to work more closely with us" from 20 foreign ministers from across Asia-Pacific who he met during his visit to Thailand last week.