LONDON — British Prime Minister Theresa May is facing fresh tensions within her Conservative Party over how to manage Britain’s departure from the European Union, distracting her from efforts to shift attention toward economic issues ahead of the party’s annual conference next week.

Foreign Secretary Boris Johnson said in comments published Saturday that a transition period to ease the impact of Brexit must not last “a second more” than the two years May proposed during a key speech in Italy last week. Johnson also said Britain must not pay for tariff-free trade with the EU, or agree to implement the bloc’s rules in order to gain such access, according to an interview with the Sun newspaper.

“There can be no monkeying around” with the transition period, Johnson said.

The comments mark the second time in less than a month that Johnson, often mentioned as a successor to May, has challenged the prime minister’s efforts to make the government appear both strong and united.

May’s performance at the party conference, which begins Sunday in Manchester, may be pivotal for her administration after a disastrous election campaign that saw the Conservatives lose their majority in the House of Commons in June.

The prime minister faced calls to discipline Johnson earlier this month after he laid out his own vision of Britain’s future outside the EU in a 4,000-word article for the Sunday Telegraph newspaper. Johnson called for Britain to adopt a low-tax, low-regulation economy outside the EU’s single market and customs union.

The latest intervention by Johnson comes as May seeks to re-establish her hold on power and turn attention toward the British economy.

“Yes, we have to get the best Brexit deal — but we must also take action here at home to make this a fairer place to live for ordinary working people,” she said. “The social contract in our country is that the next generation should always have it better than the last.”

In the months since March, when Britain triggered a two-year countdown to leave the 28-nation EU, negotiations have made little progress. The EU is demanding that Britain resolve so-called separation issues before moving onto questions about the future relationship between the two sides. Those separation issues include the status of the Ireland-Northern Ireland border and the amount Britain must pay to settle its financial commitments to the bloc.

Some leading Conservatives are suggesting Britain should end talks with the EU as early as Christmas if there is no concrete progress. Senior figures in the Leave Means Leave group say the government could instead allow World Trade Organization rules to take effect on the day Britain leaves the EU in March 2019.

“Brussels cannot be allowed to boss Britain around,” said Richard Tice, the group’s co-founder. “It’s time we showed how ready we are to walk away.”