Top Tories Boris Johnson and Greg Clark launched a Brexit charm offensive at the British Chambers of Commerce's (BCC) annual conference in central London on Tuesday (28 February).

The foreign secretary made an impassioned defence of free trade and Theresa May's "Global Britain" vision at the Westminster event.

"If we close our markets, if we put up barriers, then we raise the costs of those who can least afford it," he said.

"We make our industries uncompetitive, we entrench complacency, we discourage investment in capital and technology, we stifle innovation and, of course, we breed suspicion and mistrust between nations.

"Indeed, we should never forgot the old truths: when goods and services no longer cross borders, then troops and tanks do so instead."

The comments come just a day after former Conservative Prime Minister Sir John Major attacked May's Brexit plan, during a speech at the Chatham House think-tank.

"A little more charm, and a lot less cheap rhetoric, would do much to protect the UK's interests," he said, while branding the government's strategy as "unreal and over-optimistic".

Johnson did not mention Major by name, but urged Brexit critics to "come off it, sunshine". Clark also decided not to directly address the former Conservative leader when he was asked about Major's intervention.

The business secretary instead referenced May's now famous Lancaster House speech. "Where she set out in a cool and lucid way that our position was to have best possible trading relationship...that we should conduct the negotiation in a courteous, level-headed way, I think that is the way you can achieve an outcome that is in all of our interests," Clark said.

The BCC has urged the government to seek an extension to the two-year-long Brexit negotiations if an EU trade deal is not brokered. The request comes just weeks before May plans to invoke Article 50 of the Lisbon Treaty and trigger talks.

The former chancellor and Remain campaigner George Osborne also addressed the conference, warning that splitting from the EU's single-market, as the prime minister has promised, would be the "biggest act of protectionism" in British trade history.

Shadow Chancellor John McDonnell, meanwhile, claimed that ministers were giving the "impression of placing their narrow political positioning" above the good of the whole UK economy.

"The vote was to Leave and Labour respects that decision," he said. "But that vote to Leave is not the same as an excuse to damage the whole economy.

"It is simply not true that no deal with the EU is preferable to a bad deal. No deal is a bad deal – it's the worst possible deal, with all the dangers of WTO [World Trade Organisation]-only trading rules.

"Labour has always called for full, tariff-free access to the single-market. Nor do we think that the government should be using EU migrants as a bargaining chip."