Boris Johnson has been rapped over the knuckles by Downing Street for claiming that Britain will launch its EU exit negotiations “by the early part of next year” and that Brexit could be completed within two years.

The foreign secretary was immediately rebuffed by Theresa May, who has ordered her ministers not to engage in a “running commentary” on Britain’s negotiating strategy or the timing of Brexit talks.

Downing Street said Mrs May was not committed to any timetable for activating Article 50 of the EU treaty, which sets the clock ticking on a two-year negotiating period to complete exit talks.

Asked about Mr Johnson’s claim that Brexit could be wrapped up within two years — a view not widely shared in Whitehall — Mrs May’s spokesman said: “We would not speculate on that.”

Mr Johnson’s comments will be seen by some Tories as an attempt by the pro-Brexit wing of the party to bounce Mrs May into pressing ahead quickly with starting exit talks: the prime minister is refusing to be boxed in.

The foreign secretary has joined forces with fellow pro-Brexit ministers Liam Fox and David Davis to press for an early “clean break” with the EU, including taking Britain out of the customs union and single market.

Mr Johnson told Sky News: “We are talking to our European friends and partners in the expectation that by the early part of next year we will see an Article 50 letter.”

Mrs May has always stuck to her line that she would not trigger Article 50 before the end of 2016, but has refused to speculate on when exactly she would begin divorce proceedings.

Downing Street’s frustration was evident. “The government’s position is clear. The prime minister has said she will not trigger Article 50 before the end of the year. Ultimately it’s her decision.

“She has said she will deliver on the Brexit vote and she won’t kick it into the long grass, but she will do it when she considers she can secure the best deal for Britain.”

Last week Downing Street denied claims by Donald Tusk, European Council president, that Mrs May had told him it was “quite likely” she would activate Article 50 in “either January or February 2017”.

Mrs May is still trying to work out the government’s negotiating position and to get any sense from fellow EU leaders of their own bottom lines. Brexit talks could also be further complicated by Dutch, French and German elections next year.