The UK PM claimed this proved she needed "the strongest possible hand" in the talks.
Labour has accused the Tories of adopting a "rigid and reckless" approach to Brexit talks.
The UK is due to leave the EU by March 2019, once formal negotiations have been completed.
The government has repeatedly said it wants the terms of the separation - including demands for any exit fee - to be dealt with at the same time as a new trading relationship is negotiated.
But EU leaders have resisted this, and on Thursday Mrs Merkel told German MPs it would be "a waste of time" to maintain illusions that the two sets of negotiations could be held simultaneously.
She also said the UK could not maintain the rights it has as an EU member.
"A third country - which is what the UK will be - cannot and will not have the same rights as an EU member state," she told the Bundestag, Germany's lower house of parliament.
"All 27 EU countries and the EU institutions agree about that."
Speaking on a campaign visit to Leeds, Mrs May will repeat her claim that an increased Tory majority is needed to strengthen her Brexit negotiating hand.
"We have seen Chancellor Merkel's comments today. She says the UK has 'illusions' about the process and that the 27 member states of the European Union agree.
"We can see how tough those negotiations are going to be at times," she said.
"Yet our opponents are already seeking to disrupt those negotiations, at the same time as 27 other European countries line up to oppose us.
"That approach can only mean one thing, uncertainty and instability, bringing grave risk to our growing economy with higher taxes, fewer jobs, more waste and more debt.
"So we need the strongest possible hand, the strongest possible mandate and the strongest possible leadership as we go into those talks."
Labour set out its approach to Brexit on Tuesday, saying it would scrap Mrs May's plans and unilaterally guarantee the rights of EU residents before talks start.
Shadow Brexit secretary Sir Keir Starmer said Labour wanted a deal which prioritises jobs and workers' rights.
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