"We want to have a clear position before the European elections. In order to start a real campaign, we need to know if they will stay with us or if they will try to form another group," Mr Martens told journalists after launching the EPP manifesto for the EU elections in June.
Asked about a deadline for the Conservative leader to announce his move, Mr Martens said this should come before the EPP congress in Warsaw on 29-30 April.
When elected Tory leader in 2006, Mr Cameron had pledged to leave the umbrella-party in the parliament that brings together most of the centre-right political parties in the chamber. However, Mr Cameron has delayed making any such move until June 2009, after the EU elections.
"I understand that Mr Cameron had to make a promise to some of his members to become leader of the party. And that is his problem. He has to solve it, if he wants to stay," Mr Martens added.
The Belgian politician, who has chaired the EPP since 1990, underlined that he had "struggled a lot" to get the Tories as associate members in the European Parliament's group and that initially the co-operation was "excellent," but has deteriorated in recent years.
Mr Martens seemed confident that his party would maintain a majority in the house after the June elections, even in the case that the British delegation leaves. He pointed out that in the current legislature, the EPP group would still be in the majority even without the 27 MEPs from the UK. The EPP-ED group currently holds 288 seats, followed by the centre-left Party of the European Socialists, with 215.
According to Deutschland Radio's website, the tensions between the German and the British delegations have lately sharpened, especially on EU integration issues, where the Tories mostly vote against the group's line.
Another issue where Mr Cameron is at odds with the German conservatives is the Lisbon treaty. Strongly championed by the EPP, the Lisbon treaty would likely be scrapped if Mr Cameron won the UK general election this year. Earlier this month, he pledged to hold a referendum on the EU pact if he became prime minister. A referendum is very likely to have a negative outcome in Great Britain.
German MEP Hartmut Nassauer, head of the Christian Democrats delegation in the European Parliament, took a similar line to that of Mr Martens, calling for the Tories to make up their mind whether they want to stay or go.
"Of course we wouldn't like to lose 30-40 MEPs. But the issue with the Tories must be clarified. We won't make them any other offers to stay. They need to go through the experience of living outside the EPP, the strongest group in the Paliament," Mr Nassauer told DeutschlandRadio.
Mr Nassauer also added that the EPP would not tolerate the question remaining open, which would allow the Tories to gain key positions in the new parliament and then leave the group shortly thereafter. "This needs to be clarified before the elections," he said.