Theresa May’s claim that she will be strengthened in the Brexit talks by a general election victory has been dismissed as nonsense by the European parliament’s Brexit coordinator, who has condemned the prime minister as a political opportunist.

In an outspoken attack, Guy Verhofstadt suggests the prime minister was motivated by party political considerations rather than the national interest in calling a poll for 8 June.

Writing in the Observer, the former Belgian prime minister, who will play a key role in the coming Brexit negotiations, describes the election announced by May on Tuesday as “an attempted power grab by the Conservative party, who wish to take advantage of a Labour party seemingly in disarray to secure another five years of power, before the reality of Brexit bites”.

Verhofstadt further claims that putting more Tory MPs in the House of Commons will do nothing to bolster the British prime minister when it comes to the talks in Brussels. The latest polls have the Tories about 20 percentage points ahead of Jeremy Corbyn’s Labour party.

“The theory espoused by some, that Theresa May is calling a general election on Brexit in order to secure a better deal with the EU, is nonsensical,” he says.

“We can only conclude that many British politicians and the media still don’t fathom how article 50 will work in practice. Will the election of more Tory MPs give Theresa May a greater chance of securing a better Brexit deal? For those sitting around the table in Brussels, this is an irrelevance.”

Justifying her surprise decision to call a general election, May told the Commons last Wednesday that every vote for the Conservatives would “make me stronger when I negotiate for Britain with the European Union”.

She claimed that she needed protection from the Labour party, who had threatened to vote down a future deal; the Liberal Democrats, who wished to grind business to a halt; and the House of Lords, which has an anti-Brexit majority.

However, it has been suggested that rather than seeking to bolster herself against opponents of Brexit by gaining a larger majority, May is actually hoping to diminish the power of the hardline Brexiters in her own party, who would rather the UK crash out of the EU without a deal than see her come to a compromise.

Describing the latest developments in British politics as surreal, Verhofstadt writes: “Many in Brussels remain concerned that the chances of a deal are being eroded by the British prime minister’s tough negotiating red lines and her lack of political room for manoeuvre domestically, yet there is no guarantee that a sprinkling of additional Conservative MPs on the backbenches of the House of Commons will provide this.”

He adds: “As with the Brexit referendum, which many European leaders saw as a Tory cat-fight that got out of control, I have little doubt many on the continent see this election as once again motivated by the internal machinations of the Tory party.”

With reference to a TV clip of a dismayed British voter being told of another election in the UK, which went viral on the internet last week, he writes: “The BBC video of Brenda from Bristol, so openly decrying another political campaign, was viewed far beyond the white cliffs of Dover. Indeed, it appears this election is being driven by the political opportunism of the party in government, rather than by the people they represent.”

Verhofstadt strikes a pessimistic note about the Brexit talks, which will now take place after the UK’s general election in June. He warns that, as it stands, “unless the UK government requests transitional arrangements to the contrary, and these requests are agreed by all EU countries, UK citizens will have no more of a right to holiday, travel and study in EU countries than tourists from Moscow or students from Mumbai”.

He also lambasts David Davis’s Department for Exiting the European Union for claiming that the two EU agencies currently based in London – the European Banking Authority and the European Medicines Agency – might stay in the UK after Brexit. The department made the claim after this newspaper revealed that the selection criteria for the coming contest between member states seeking to gain the agencies are to be published at the end of this month.

Verhofstadt writes: “As the Observer has reported, leaving the European Union means the EU agencies based in the United Kingdom will be relocated.

“I expect this will be approved by EU leaders as soon as June, if not before. Contrary to the obscure claims by UK government officials, the EU’s ‘crown jewels’ of the European Banking Authority and the European Medicines Agency will not remain in a post-Brexit Britain, paid for by EU countries.

“This is not, as the Daily Express has already decried, a ‘punishment’. This is another logical consequence of Theresa May’s article 50 letter. This decision will not be up for negotiation.”

The president of the European commission, Jean-Claude Juncker, will be meeting May in Downing Street on Wednesday to discuss the process for the two years of negotiations allowed under article 50 of the Lisbon treaty.

The EU will formalise its broad political goals for the negotiations at a summit next Saturday in Brussels. The more detailed European commission directives for its chief negotiator, Michel Barnier, will be adopted at the end of May, after which the EU will be ready to start talks, with citizens’ rights, the UK’s financial liabilities and the border in Ireland the priorities.

Keir Starmer, the shadow brexit minister, said: “Guy Verhofstadt asks, ‘what is the purpose of this general election’? The answer is simple. The prime minister is attempting to crush all challenge to her hard Tory Brexit approach at home and to negotiate by threat and demand abroad. As Guy Verhofstadt rightly points out, far from helping negotiations with the EU, the prime minister’s stance is eroding the chances of achieving the best deal for Britain.”