The National Health Service has become a battleground in the election as Labour candidates have accused the Conservative Party of trying to sell it off as part of a trade deal with the United States. Mr. Corbyn said the health agency would be excluded from any trade deals if he won.

Mr. Johnson pledged to lower business tax rates — those taxes based on the size of property a company occupies — in an effort to stimulate small and medium-size businesses.

The British Retail Consortium, representing man y shop owners , welcomed the announcement, but said it needed more from the government. The announced tax rate cuts, said Helen Dickinson, the group’s chief executive, would not slow the economic decline in some towns “that has seen many household names disappear in recent years.”

Mr. Johnson insisted that his party would seal the deal on Brexit, while accusing Mr. Corbyn of delaying Britain’s departure.

“The only hard fact that has swum like flotsam from the Bermuda Triangle of Labour’s Brexit policy is that they want more delay,” Mr. Johnson said. Mr. Corbyn has said he would renegotiate the deal and then allow the public to vote on it, a position that Mr. Johnson said was like “dying Virgil urging his friends to burn ‘The Aeneid,’” an ancient literary allusion that was met with silence at the forum.

Mr. Corbyn, for his part, tried to repair any sense that his policies have caused resentment in the corporate community.

“It is sometimes claimed that I am anti-business,” he said. “Actually this is nonsense.”

Mr. Corbyn appealed to the attendees by promising the “certainty of a customs union and access to the single market” after Brexit, and said he would wrap up Brexit negotiations quickly.