The head of the European Parliament on Thursday vowed "a wide-ranging reform package" to clean up the legislature amid a graft scandal linked to World Cup host Qatar.

Roberta Metsola said the plan "will include a strengthening of the parliament's whistle-blower protection systems, a ban on all unofficial friendship groups, a review of the policing of our code of conduct rules, and a complete and in-depth look at how we interact with third countries".

She said she would lead the changes, and that the package would be ready "in the new year".

Belgian authorities triggered the scandal by detaining six people last week.

Four of them - including an MEP and former European Parliament vice president under Metsola, Eva Kaili - have been charged with "criminal organisation, corruption and money laundering". The other two were released.

Both Kaili, a former TV newsreader in Greece, and Qatari officials deny any wrongdoing.

A series of searches at the homes and offices of politicians, lobbyists and parliamentary assistants turned up around €1.5 million in cash.

A Belgian judicial source said €600,000 were found at the home of an MEP-turned-lobbyist, Pier Panzeri, €150,000 in Kaili's flat and €750,000 in her father's hotel room.

The probe and the charges have shaken the European Parliament, and put its interactions with lobbyists and representatives of non-EU countries under intense scrutiny.

Metsola has sought to frame the scandal as an outside attack on "European democracy" and said that take was endorsed by EU leaders who gathered for a Brussels summit on Thursday.

She also emphasised that the investigation was being carried out by Belgian law enforcement authorities, and the parliament was giving them "full and open cooperation".

But she acknowledged that "the rules that we have already can be tightened can be improved" when it came to monitoring transactions and trips by the parliament's 705 lawmakers, and the lobbying efforts they were subject to by non-EU countries.

"Certain things that happened in this context will not be allowed to happen again," she said.