Britney Spears has missed eight out of 14 court-ordered drug tests, her ex-husband's lawyer has claimed.

The troubled star - who lost custody of her two sons, Sean Preston, two, and 13-month-old Jayden James, to Kevin Federline last month - has failed to respond to several calls requesting she take the tests within an hour.

Mark Vincent Kaplan said: "We have been in court on this exact issue at least three times.

"She lives in a parallel universe. Mr. Federline doesn't want to take these children out of their mother's life, but what are we to do?"

A lawyer for Britney - who was ordered by a judge to undergo random drug tests to prove she's a suitable mother - said the tests are
"unconstitutional".

Anne Kiley added: "Since October 5 there have been 10 negative drug tests, the idea that she needs to respond within the hour is meaningless.

"Is the issue that Britney Spears needs to get up at 8.30am and answer the phone? She's a pop star with a number one album. She is not up that early."

However, Judge Scott Gordon hit back, saying the time of the tests are important as it will prove whether the 'Toxic' singer can follow rules.

He said: "She needs to take responsibility. The issue is to see if she can follow a court order.

"Since we started this, there have been lost phones, changed numbers,
voicemails being left on someone else's voicemail.

"Maybe we should set up a fixed appointment for her to be tested. The point is we cannot continue to do these weekly ex-parte motions."

Britney currently has three supervised visits a week with her sons but last month Judge Gordon took her rights away after she couldn't be contacted on a mobile phone number she had given.

He later reinstated her visitation rights when she gave the correct number.

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