Theresa May will today warn Angela Merkel that she needs time to prepare for Brexit, as she heads to Berlin for private talks with the German chancellor.

The new prime minister will fly to Berlin after facing her first prime minister’s questions against Labour leader Jeremy Corbyn on Wednesday.

Merkel and May’s working dinner is likely to be dominated by discussion of Britain leaving the EU but they are also likely to touch on the trading relationship between the two countries, the migration crisis and Islamist terrorism.

Her meeting with Merkel will be followed by talks in Paris on Thursday with the French president, François Hollande. During both discussions, the prime minister is likely to warn that the UK needs time to consult with Scotland, Wales and Northern Ireland as well as different industries, before triggering formal negotiations to leave the EU.

At the opening of the first legal challenge to the process of Brexit on Tuesday, government lawyers confirmed that May will not push the button on article 50 of the Lisbon treaty, which initiates the UK’s departure, before the end of this year.

Sources said May will be hoping to establish personal relations with both Merkel and Hollande to pave the way for open and frank discussions in the months ahead, suggesting she expects much work towards Brexit to be done through diplomacy with individual leaders as well as in talks with the EU itself.

Before the trip, the prime minister, who campaigned to remain in the EU, said she had chosen to visit Berlin and Paris so soon after assuming office because she was determined to make a success of Brexit.

“These visits will be an opportunity to forge a strong working relationship that we can build upon and which I hope to develop with more leaders across the European Union in the weeks and months ahead,” she said. “I do not underestimate the challenge of negotiating our exit from the European Union and I firmly believe that being able to talk frankly and openly about the issues we face will be an important part of a successful negotiation.”

Earlier on Tuesday, May revealed she will personally take charge of three new ministerial committees – on Brexit, the economy and social reform – to implement her priorities for government. No 10 denied it was a move to keep tight control on the policy areas covered by Boris Johnson, the foreign secretary, David Davis, the Brexit secretary, and Liam Fox, the international trade secretary.

The prime minister told her first cabinet meeting that “politics is not a game” and they must get on with the job.

“Brexit means Brexit – and we’re going to make a success of it. It will be the responsibility of everyone sitting around the cabinet table to make Brexit work for Britain,” she said before the meeting.

“And it will also be their duty to deliver success on behalf of everyone in the UK, not just the privileged few. That is why social justice will be at the heart of my government.

“So we will not allow the country to be defined by Brexit; but instead build the education, skills and social mobility to allow everyone to prosper from the opportunities of leaving the EU.”

Separately, it emerged that Nick Clegg, the Lib Dem former deputy prime minister, will be returning to frontline politics as his party’s spokesman on the EU.

“Theresa May says Brexit means Brexit, but no one actually knows what that means. Will we be in the single market or cut off from it, with all the implications that has for British jobs and our economy?” he said.

“What does it mean for immigration? What about the Brits who live abroad and the Europeans who have made our country their home? How will we co-operate with our neighbours to tackle terrorism, cross-border crime and climate change?”