Guy Verhofstadt said there was no guarantee additional Conservative MPs at Westminster would give the Prime Minister more room for manoeuvre in the talks, as some observers have argued.
Writing in The Observer, the former Belgian prime minister said Mrs May's decision to call a snap poll on June 8 - having previously said she would not go to the country before the scheduled end of the parliament in 2020 - appeared to have been driven by "political opportunism".
His comments are likely to irritate Mrs May, who has argued that an increased Commons majority would strengthen her hand in the Brexit talks, making it more difficult for the opposition parties at home to obstruct her plans.
In his article, Mr Verhofstadt wrote: "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.
"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.
"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.
"Indeed, it appears this election is being driven by the political opportunism of the party in government, rather than by the people they represent."